diff --git a/src/assembler.c b/src/assembler.c
index d42e6be..31a1cdf 100644
--- a/src/assembler.c
+++ b/src/assembler.c
@@ -44,7 +44,7 @@ static ErrorInfo* resolve_defaults(AssemblerState *state)
                     // check reported rom size is <= actual rom size
 
     // if (!state.header.rom_size)
-            // set to actual rom size
+            // set to actual rom size using util's size_bytes_to_code()
 
     (void) state;
     return NULL;
diff --git a/src/assembler/preprocessor.c b/src/assembler/preprocessor.c
index 80abfc3..a7de4c0 100644
--- a/src/assembler/preprocessor.c
+++ b/src/assembler/preprocessor.c
@@ -306,6 +306,14 @@ static ErrorInfo* build_asm_lines(
 }
 
 /*
+    Return whether the given header offset is a valid location.
+*/
+static inline bool is_header_offset_valid(uint16_t offset)
+{
+    return offset == 0x7FF0 || offset == 0x3FF0 || offset == 0x1FF0;
+}
+
+/*
     Parse the region code string in an ASMLine and store it in *result.
 
     Return true on success and false on failure; in the latter case, *result is
@@ -334,11 +342,20 @@ static bool parse_region_string(uint8_t *result, const ASMLine *line)
 }
 
 /*
-    Return whether the given header offset is a valid location.
+    Parse the size code in an ASMLine and store it in *result.
+
+    Return true on success and false on failure.
 */
-static inline bool is_header_offset_valid(uint16_t offset)
+static bool parse_size_code(uint8_t *result, const ASMLine *line)
 {
-    return offset == 0x7FF0 || offset == 0x3FF0 || offset == 0x1FF0;
+    uint32_t bytes;
+    if (!parse_uint32_t(&bytes, line, DIR_ROM_DECLSIZE))
+        return false;
+
+    uint8_t code = size_bytes_to_code(bytes);
+    if (code)
+        return (*result = code), true;
+    return false;
 }
 
 /*
@@ -417,8 +434,12 @@ ErrorInfo* preprocess(AssemblerState *state, const LineBuffer *source)
         END_DIRECTIVE
 
         BEGIN_DIRECTIVE(DIR_ROM_DECLSIZE, uint8_t, state->header.rom_size, 0)
-            // TODO: fixme
-            FAIL(ED_PP_UNKNOWN)
+            PARSER_BRANCH(uint8_t, {
+                CLAMP_RANGE(0x10)
+                VALIDATE(size_code_to_bytes(arg))
+            }, {
+                VALIDATE(parse_size_code(&arg, line))
+            })
         END_DIRECTIVE
 
         END_DIRECTIVE_BLOCK
diff --git a/src/rom.c b/src/rom.c
index 34fcbfe..4fa0e5b 100644
--- a/src/rom.c
+++ b/src/rom.c
@@ -16,6 +16,7 @@
 #define NUM_LOCATIONS 3
 #define MAGIC_LEN 8
 #define HEADER_SIZE 16
+#define SIZE_CODE_BUF 8
 
 static size_t header_locations[NUM_LOCATIONS] = {0x7FF0, 0x3FF0, 0x1FF0};
 static const char header_magic[MAGIC_LEN + 1] = "TMR SEGA";
@@ -100,18 +101,17 @@ static uint16_t compute_checksum(const uint8_t *data, size_t size, uint8_t range
 */
 static const char* parse_reported_size(uint8_t value)
 {
-    switch (value) {
-        case 0xA: return "8 KB";
-        case 0xB: return "16 KB";
-        case 0xC: return "32 KB";
-        case 0xD: return "48 KB";
-        case 0xE: return "64 KB";
-        case 0xF: return "128 KB";
-        case 0x0: return "256 KB";
-        case 0x1: return "512 KB";
-        case 0x2: return "1 MB";
-        default:  return "Unknown";
-    }
+    static char buffer[SIZE_CODE_BUF];
+    size_t size = size_code_to_bytes(value);
+
+    if (!size)
+        strncpy(buffer, "Unknown", SIZE_CODE_BUF);
+    else if (size >= (1 << 20))
+        snprintf(buffer, SIZE_CODE_BUF, "%zu MB", size >> 20);
+    else
+        snprintf(buffer, SIZE_CODE_BUF, "%zu KB", size >> 10);
+
+    return buffer;
 }
 #endif
 
diff --git a/src/util.c b/src/util.c
index bb90c66..857bcb0 100644
--- a/src/util.c
+++ b/src/util.c
@@ -90,3 +90,54 @@ uint8_t region_string_to_code(const char *name)
     }
     return 0;
 }
+
+/*
+    Return the number of bytes in a ROM image based on its header size code.
+
+    0 is returned if the code is invalid.
+*/
+size_t size_code_to_bytes(uint8_t code)
+{
+    #define KB << 10
+    #define MB << 20
+
+    switch (code) {
+        case 0xA: return   8 KB;
+        case 0xB: return  16 KB;
+        case 0xC: return  32 KB;
+        case 0xD: return  48 KB;
+        case 0xE: return  64 KB;
+        case 0xF: return 128 KB;
+        case 0x0: return 256 KB;
+        case 0x1: return 512 KB;
+        case 0x2: return   1 MB;
+        default:  return   0;
+    }
+
+    #undef KB
+    #undef MB
+}
+
+/*
+    Given the number of bytes in a ROM image, return the size code.
+
+    0 is returned if the size is invalid.
+*/
+uint8_t size_bytes_to_code(size_t bytes)
+{
+    if (bytes & ((1 << 10) - 1))
+        return 0;  // Not an even number of KB
+
+    switch (bytes >> 10) {
+        case    8: return 0xA;
+        case   16: return 0xB;
+        case   32: return 0xC;
+        case   48: return 0xD;
+        case   64: return 0xE;
+        case  128: return 0xF;
+        case  256: return 0x0;
+        case  512: return 0x1;
+        case 1024: return 0x2;
+        default:   return   0;
+    }
+}
diff --git a/src/util.h b/src/util.h
index 539cfba..b82df0c 100644
--- a/src/util.h
+++ b/src/util.h
@@ -11,3 +11,5 @@ uint8_t bcd_decode(uint8_t);
 uint64_t get_time_ns();
 const char* region_code_to_string(uint8_t);
 uint8_t region_string_to_code(const char*);
+size_t size_code_to_bytes(uint8_t);
+uint8_t size_bytes_to_code(size_t);