2-3 months old could be right-but she could be older if the care she had was poor. IF she is that young, a 10 gallon would suffice for the time being, however chameleons do not belong in tanks. Soon you will need an appropriate screen cage. In the 10 gallon-you need to be extra careful not to bake your chameleon. A digital thermometer in the hottest spot should not read higher than 80, and the other side of the tank should be in the low 70's. You need to mist enough to get her to drink, and water build up will be an issue. The tank should never have standing water, babies can drown, and stagnant water and heat is a recipe for respiratory infections. Please tell us what kind if lighting you are using.