As big as you feel is necessary. You can always setup another 10 gallon if you feel the one is getting crowed. They have to do quite a bit of growing before the parents, or other fish, are able to eat them. At .5" they should be big enough to be left alone.
The bigger the tank, the faster they will grow...(this is the case for all fish)...it depends on how long it'll be before they outgrow the 10g but I would say around 4-6 months