well for one i wouldnt recommend getting a viv for a gecko... if it is a gecko with claws instead of sticky pads, such as a leapord gecko i would keep it in a faunarium with sand & a hide/plants etc... and this will do for any size gecko...
Whereas if you are getting a gecko with sticky feet, such as a Gargoyle Gecko I would recommend a terrarium that is around 18'' wide, 18" deep and about 2 foot high with plenty of coverage and plants and climbing materials (wood, vine, bark etc...) unless it is going to grow 8"+ then i would recommend a bigger terrarium...
1.0.0 Red Amelanistic Corn - Diablo
0.1.0 Okatee Corn - Akeyra
0.1.0 Anerythristic Corn - Seraphis
1.0.0 Normal Royal Python - Kirby
1.0.0 Crested Gecko - Roger