I don't know if a fig tree grown indoors ever has a dormant period, but I know that the one I used to have in the garden could only be pruned when it was dormant and before it came into leaf. They exude a white sap which, if you prune the tree at the wrong time, runs from it like a river, which can't be good for the tree. It will still produce this, even when dormant, but not in such huge quantities. You will probably have to take a year or three to gradually reduce the tree to a more manageable size.