diff --git a/src/assembler.c b/src/assembler.c
index 44c1045..15bcb80 100644
--- a/src/assembler.c
+++ b/src/assembler.c
@@ -9,6 +9,8 @@
 #include "assembler/preprocessor.h"
 #include "assembler/state.h"
 #include "logging.h"
+#include "rom.h"
+#include "util.h"
 
 /*
     Tokenize ASMLines into ASMInstructions.
@@ -48,17 +50,21 @@ static ErrorInfo* tokenize(AssemblerState *state)
 */
 static ErrorInfo* resolve_defaults(AssemblerState *state)
 {
-    // TODO
+    if (!state->rom_size) {
+        state->rom_size = 32 << 10;
 
-    // if (!state.rom_size)
-            // set to max possible >= 32 KB (max of instructions, offset, and state.header.rom_size)
+        // TODO: use highest instruction too
 
-    // if (!state.header.rom_size)
-            // set to actual rom size using util's size_bytes_to_code()
+        if (state->header.rom_size != INVALID_SIZE_CODE) {
+            size_t decl_size = size_code_to_bytes(state->header.rom_size);
+            if (decl_size > state->rom_size)
+                state->rom_size = decl_size;
+        }
+    }
 
-    state->rom_size = 8;
+    if (state->header.rom_size == INVALID_SIZE_CODE)
+        state->header.rom_size = size_bytes_to_code(state->rom_size);
 
-    (void) state;
     return NULL;
 }
 
diff --git a/src/assembler/parse_util.c b/src/assembler/parse_util.c
index 3f89355..26a2600 100644
--- a/src/assembler/parse_util.c
+++ b/src/assembler/parse_util.c
@@ -202,7 +202,7 @@ PARSE_FUNC_HEADER(size_code, uint8_t)
     }
 
     uint8_t code = size_bytes_to_code(bytes);
-    if (code)
+    if (code != INVALID_SIZE_CODE)
         return (*result = code), true;
     return false;
 }
diff --git a/src/assembler/preprocessor.c b/src/assembler/preprocessor.c
index bf41671..ab06ee4 100644
--- a/src/assembler/preprocessor.c
+++ b/src/assembler/preprocessor.c
@@ -51,7 +51,7 @@
     if (CALL_PARSER_(arg_type)) {true_part} else {false_part}
 
 #define SAVE_LINE(target)                                                     \
-    target = line;
+    if (!dir_is_auto) target = line;
 
 #define BEGIN_DIRECTIVE_BLOCK                                                 \
     ssize_t first_ctr = -1;                                                   \
@@ -63,7 +63,8 @@
         FAIL_ON_COND_(!DIRECTIVE_HAS_ARG(line, directive), ED_PP_NO_ARG)      \
         arg_type arg = 0;                                                     \
         arg_type* dest = &(dest_loc);                                         \
-        if (DIRECTIVE_IS_AUTO(line, directive)) {                             \
+        bool dir_is_auto = DIRECTIVE_IS_AUTO(line, directive);                \
+        if (dir_is_auto) {                                                    \
             arg = auto_val;                                                   \
         } else {
 
@@ -311,6 +312,14 @@ static ErrorInfo* build_asm_lines(
 }
 
 /*
+    Return whether the given ROM size is valid.
+*/
+static inline bool is_rom_size_valid(size_t size)
+{
+    return size_bytes_to_code(size) != INVALID_SIZE_CODE;
+}
+
+/*
     Return whether the given header offset is a valid location.
 */
 static inline bool is_header_offset_valid(uint16_t offset)
@@ -365,7 +374,7 @@ ErrorInfo* preprocess(AssemblerState *state, const LineBuffer *source)
             PARSER_BRANCH(uint32_t, {}, {
                 USE_PARSER(uint32_t, rom_size)
             })
-            VALIDATE(size_bytes_to_code)
+            VALIDATE(is_rom_size_valid)
             SAVE_LINE(rom_size_line)
         END_DIRECTIVE
 
@@ -397,7 +406,7 @@ ErrorInfo* preprocess(AssemblerState *state, const LineBuffer *source)
             })
         END_DIRECTIVE
 
-        BEGIN_DIRECTIVE(DIR_ROM_DECLSIZE, uint8_t, state->header.rom_size, 0)
+        BEGIN_DIRECTIVE(DIR_ROM_DECLSIZE, uint8_t, state->header.rom_size, DEFAULT_DECLSIZE)
             PARSER_BRANCH(uint8_t, {
                 CHECK_RANGE(0x10)
                 VALIDATE(size_code_to_bytes)
@@ -416,18 +425,21 @@ ErrorInfo* preprocess(AssemblerState *state, const LineBuffer *source)
         line = prev;
     }
 
-    if (state->rom_size && state->header.offset + HEADER_SIZE > state->rom_size) {
+    if (rom_size_line && state->header.offset + HEADER_SIZE > state->rom_size) {
         ei = error_info_create(rom_size_line, ET_PREPROC, ED_PP_HEADER_RANGE);
         goto cleanup;
     }
 
-    if (state->rom_size && state->header.rom_size &&
+    if (rom_size_line && rom_declsize_line &&
             size_code_to_bytes(state->header.rom_size) > state->rom_size) {
         ei = error_info_create(rom_size_line, ET_PREPROC, ED_PP_DECLARE_RANGE);
         error_info_append(ei, rom_declsize_line);
         goto cleanup;
     }
 
+    if (!rom_declsize_line)  // Mark as undefined, for resolve_defaults()
+        state->header.rom_size = INVALID_SIZE_CODE;
+
     cleanup:
     asm_lines_free(condemned);
     state->lines = dummy.next;  // Fix list head if first line was a directive
diff --git a/src/assembler/state.c b/src/assembler/state.c
index 5b79fde..a62e97f 100644
--- a/src/assembler/state.c
+++ b/src/assembler/state.c
@@ -17,7 +17,7 @@ void state_init(AssemblerState *state)
     state->header.product_code = 0;
     state->header.version = 0;
     state->header.region = DEFAULT_REGION;
-    state->header.rom_size = 0;
+    state->header.rom_size = DEFAULT_DECLSIZE;
     state->optimizer = false;
     state->rom_size = 0;
 
diff --git a/src/assembler/state.h b/src/assembler/state.h
index 477186a..de0c2c5 100644
--- a/src/assembler/state.h
+++ b/src/assembler/state.h
@@ -11,6 +11,7 @@
 
 #define DEFAULT_HEADER_OFFSET 0x7FF0
 #define DEFAULT_REGION 6  // GG Export
+#define DEFAULT_DECLSIZE 0xC  // 32 KB
 
 #define SYMBOL_TABLE_BUCKETS 128
 
diff --git a/src/util.c b/src/util.c
index 857bcb0..050c7a9 100644
--- a/src/util.c
+++ b/src/util.c
@@ -121,7 +121,7 @@ size_t size_code_to_bytes(uint8_t code)
 /*
     Given the number of bytes in a ROM image, return the size code.
 
-    0 is returned if the size is invalid.
+    INVALID_SIZE_CODE is returned if the size is invalid.
 */
 uint8_t size_bytes_to_code(size_t bytes)
 {
@@ -138,6 +138,6 @@ uint8_t size_bytes_to_code(size_t bytes)
         case  256: return 0x0;
         case  512: return 0x1;
         case 1024: return 0x2;
-        default:   return   0;
+        default:   return INVALID_SIZE_CODE;
     }
 }
diff --git a/src/util.h b/src/util.h
index b82df0c..237ad21 100644
--- a/src/util.h
+++ b/src/util.h
@@ -5,6 +5,8 @@
 
 #include <stdint.h>
 
+#define INVALID_SIZE_CODE 0x8
+
 /* Functions */
 
 uint8_t bcd_decode(uint8_t);