I would make the assumption that, in the CoCo 3's case, a 32k, 64k or 128k ROM can be created.
According to the discussion on the CoCo Mailing list, the normal size is a little less than 16k or 32k. Any carts that require larger ROMS need to have hardware MMU's to take advantage of a bank-switching scheme. There's a seller on eBay who sells a 512k cart with a control pad, to determine which bank on the ROM the CoCo will boot to. The same could be done with software to read the data from the different banks, in effect creating a larger game or application.
Additionally, if you get to a point where you need a larger ROM, you could look at the content of the code; if there's a lot of graphics or constant data, you could compress the code, and decompress it in memory. Not much different than some disk games load the data and then decompress in memory (Sock Master's Donkey Kong for example).
Your fictitious 256k cart could be done with an add-on MMU (like Robocop used for it's 128k) to handle the bank switching. As one list member said, there doesn't appear to be any such hardware under development for this right now, but, it has been done.