How big can a Madagascar day gecko become?

All Day Geckos of the genus Phelsuma come from Madagascar.

Most likely, you are referring to the Madagascar Giant Day Gecko (Phelsuma madagascariensis madagascariensis).

Madagascar giant day geckos may reach up to 22 cm (8.5 inches) in length from nose to tail tip.

Another commonly kept species was formerly called Phelsuma madagascariensis grandis, but is now Phelsuma grandis. Known in the trade as Grandis day geckos, they can reach up to 12 inches in length, and they are the largest of the day geckos, so this should answer your question either way.