It's actually a topic that's gotten some discussion over in the
Superguide over the years (as well as
shopping for what you need, not what you want) for what it's worth... but it's not in the core seven posts.
2FF mini SIM is pretty standard with feature phones and even most older smartphones. 2FF SIM cards can also be cut down to 3FF micro easily either manually or with a cheap punch, and can be easily adapted back up to 2FF size with plastic adapters from Noosy as well.
Technically, AT&T GoPhones can and will work with AT&T postpaid accounts, you just chuck your active SIM card into the phone. These days, you might have trouble getting AT&T to carrier unlock GoPhones on the postpaid end, however.
The best phone is a carrier unlocked one so you don't necessarily have to stick with AT&T specifically and can potentially go to cheaper GSM MVNO options if desired. Plenty of feature phones are available new, used and refurbished on Amazon and Ebay, including various trusty rusty Nokia candy bar beaters and some reasonably priced IP67 rated Samsung feature phones. If you're looking for specific features with a handset,
try here. The thing to remember is that US GSM bands (AT&T) are 850/1900. You can find a whole range of phones from $25 to $250 in this category, with features ranging from IP67 ruggedization to two SIM card slots to giant old people buttons and interfaces. There's no shortage.
I've got a short list of reasonably decent phones spreading the gamut from basic feature phones to smartphones over at the
Shopping Hut which is my Amazon Referral Program set up to help support the
Superguide and
Technical Meshugana, and their further research, expansion and availability. Use it or don't, no skin off my nose... it's just there if you find it handy.