Hi Steve,
You can create as large an epoxy sphere as you want, provided you follow the ‘rules’ of the resin you are using. This means particularly paying attention to how much resin you can mix and pour at once and in how deep a layer.
The best resin to make your epoxy sphere is the Resin Obsession deep pour resin. You can mix anywhere from 3 ounces to 3 gallons at once, but pour in a depth in no more than two inches at once, otherwise, the resin may overheat and crack. You can buy the resin in our store here: https://shop.resinobsession.com/collections/resin/resin-obsession-deep-pour-resin
Unfortunately though, all resins are going to yellow with time. Resins with UV inhibitors and stabilizers help extend the time before yellowing occurs. However, things such as mixing off ratio (even though it still can cure without tackiness), putting over an improperly cured lacquer, excessive heat, and or constant exposure to sunlight can speed up the yellowing process. How long the epoxy will last in its clear state is very difficult to predict.
We have sphere molds in our store here: https://shop.resinobsession.com/collections/molds/sphere