Just a reminder that providing specifics on, sharing links to, or naming websites where ROMs can be accessed is against the rules. If your post has any of this information it will be removed.
Welcome to PokéCommunity! Register now and join one of the best fan communities on the 'net to talk Pokémon and more! We are not affiliated with The Pokémon Company or Nintendo.
XSE has an option somewhere to expand the ROM for you. Alternatively, you can use a hex editor; just paste insert 16MB of 0xFF bytes at the end of the ROM (easiest way to do this is to just copy and paste the entire ROM, then fill the last 16MB with free space bytes).