As this page (http://www.retrode.org/documentation/the-retrode-cfg-file/) says, the force size for gboy mode (gb/gbc) is limited to "1-128 (=16kByte banks)". Unfortunately, that is not the actual maximum size of a GBC cart. The biggest GBC game is Densha De Go 2, which is a 64Mbit (8MB) game. So, it actually has 512 16kByte banks. Even without that extreme example though, there are several 4MB games in Europe/US. These dump just fine when correctly auto-detected, but you can't force them to dump correctly. Unfortunately, one of my games (Cannon Fodder) is incorrectly auto-detected as 2Mbit (256KB), making it impossible to dump it correctly :( Would be great if the maximum for the config could be increased.
Edit: Cannon Fodder now is autodetected correctly. Still, the request above stands, one should be able to force up to 64Mbit
Edit2: I see a similar problem with the SNES looking at the config documentation again. Force max is 32Mbit, but actually the maximum supported LoROM size was 64Mbit. I don't think any official games go beyond 48Mbit, but it should still be possible to force 64Mbit.