That's a really good point, although I've had good experience with COA in that regard. After placing an order recently Dan informed me that the Parachromis I ordered were smaller than what he had listed, and he offered to provide me with several extras to compensate. I was OK with that because I agreed to it in advance and had the space. A similar situation came up with Cichlid Connection a while back, but in that case Gage just sent me three 1.5" P. synspilum instead of the 3-4" fish I ordered (along with other stuff, including a large male freddy when I ordered a female). In the vendor's mind it was a good tradeoff (more fish), but for me it was a problem of finding space for the small ones and not having something large enough to go in the tank I had planned for. When placing orders I now ask for confirmation about size so I can adjust my order if necessary. I'd rather the vendor be upfront if they don't have what advertised rather than send me a solution they think should be acceptable.
I'd give COA another chance because he does sell top-notch fish. Just prompt Dan for confirmation about size so that he knows it is an important part of the sale condition.