A photo would help. You must judge the health of the tree before defoliating, as defoliation is a major stress for the tree.
You are in Washington DC, which is only suitable climate for Ficus for about 6 months out of the year. How you winter your Ficus will also determine how often it can be defoliated. Even in South Florida, they seldom defoliate more than twice a year, in Washington DC, I would think once a year might be all you can get away with. Repeated defoliation can slowly weaken a tree, sending it into a decline that potentially could result in dead tree. Ficus only seem to be indestructible, in reality, it is possible to kill one.
Defoliation alone will only somewhat reduce leaf size. The single most effective way to decrease leaf size is to increase ramification. The more branches your branches have. The more degrees of branching, ramification, the smaller the leaves.
Instead of defoliation, I would prune the growing ends of all branches. Force them to branch if they are to continue to grow. Depending on the health of the tree, you keep most of the leaves, or you defoliate at the same time as you prune your branches. It is the pruning that gets the results, not the defoliation. You can keep old leaves, and the new will be smaller. Later, you eventually remove the older larger leaves, but by keeping them until the new branches have leafed out, you avoid over-stressing the tree.
But it is the pruning of the branches that decreases leaf size, because it causes additional branching. Defoliation alone will not significantly reduce leaf size.
Jerry Meislik wrote a good book on Ficus for bonsai, it is still in print available through Stone Lantern Publishing. You should seek it out, it is not expensive.