You shouldn't keep them at all IMO
The question was what size of tank, not IF...
And why should one not be keeping them? I mean assuming you live in a jurisdiction where they are not illegal? There isn't conclusive evidence to support the "usual" reasons -
1) they're not painted or dyed (so the painted glassfish reasons fail there)
2) there isn't any conclusive evidence to prove any damage to the environment if they are released
3) they aren't emiting any toxins into the aquarium
4) the fish itself isn't functionally disabled like some "natural" goldfish
A GM fish is just like any GM product - I don't see how it's any worse than modified corn - built for a purpose (see the glo-fish website)
Just my two cents