Rom Expansion Explained

This utility makes your rom bigger or smaller. GBA roms are usually between 16 MB and 32 MB (0x1000000 and 0x2000000 bytes). You can specify any hexadecimal (base 16) number between 0x1000000 and 0x2000000 to assign a new file size in bytes. Alternatively, you can type any decimal (base 10) number between 16 and 32 to specify the number of megabytes the ROM will have.

Warning: If you specify a number of bytes/megabytes that's less than the current ROM's file size, then the file will be truncated, and you will lose whatever data got truncated. Having a backup of your ROM before truncating your ROM would be wise.

... What else do you want?