Sometimes baby bettas get stressed out when they are in too large of a tank. Of course this does not go for all of them- some do fine.
Typically people put their baby betta in a 2.5 gallon and sometimes upgrade once the betta is bigger.
IMO you can go with a big tank, but try to fill it decently and watch her behavior to make sure she does not feel stressed.
Since she has been through a lot with ammonia, IMO you should get your bigger tank and fish less cycle it, then add her when the cycle is finished and you get a 0 ammonia 0 nitrite and small nitrate reading.
A 2.5 gallon tank is difficult to keep a stable cycle with, so it is something that would require two weekly water changes- one 50% change and one 100% change. If you go with something 5-10 gallons, it would need once a week water changes. If you do not cycle the tank, it will eventually need a 100% water change to rid of excess ammonia and prevent ammonia build up, so cycling the tank is highly advisable. A fully cycled tank 5-10 gallons would need a once a week 25-30% water change, and NEVER a 100% change.
here is a good resource on baby bettas: http://bettacarecentral.weebly.com/baby-betta-care.html
Good resource on fish less cycling: http://www.bettafish.com/showthread.php?t=111960