I think you could get away with a 2.5g tank for the initial QT period, provided you were very on top of maintaining water quality - but for anything longer I'd say 5g minimum. . . 'long' not 'tall'
To clarify, if the frogs have been kept with fish, you'll want to keep them in QT for a period of at least one month to ensure that they aren't 'carrying' anything that could be harmful to fish. This would be by association, as ADF don't typically become infected by the same pathogens that would affect fish. They can *happen* to have picked up fish parasites that can't use them as a host, but can 'hitchhike' in on their skin, water from the shop, or internally - having been ingested with or as food.
IF you get the two frogs at separate times, you will want to QT the second arrival for a period of 3 months, because frogs can be infected with the Chytrid fungus, which can take as long as 12 weeks to manifest, and almost always kills - but doesn't affect fish. I'd recommend that you purchase both frogs at the same time to keep things simple, as you will have to be VERY careful to keep the tank and supplies (buckets, siphons, nets) entirely separate during the QT period.
Hope this helps?