I agree with
@Lutonian Grow a trunk before trying for small leaves. Any leaf reduction will slow trunk growth a lot.
When it is time for smaller leaves arnification is your best option. The more growing points a tree has the more leaves there will be and each leaf will be correspondingly smaller.
Figs do respond to defoliation but take care. One of the reasons for smaller leaf after defoliation is lack of energy and resources. It is possible to really hurt a tree by repeated defoliation. Leaf size after a first defoliation may not reduce much but with luck you will get some added ramification which will help in later years.
Another strategy for leaf reduction on any ficus is to use its natural growth habit. Check a ficus shoot as it grows. The first leaf to open is small. Each succeeding leaf is a little larger so for smaller leaves on better developed trees just pinch the growing tip out after the first leaf has unrolled. A new shoot will emerge from axillary bud at the base of the leaf and the new shoot will start with a small leaf. Continue ad infinitum.
You can also remove the sheath that protects the growing terminal shoot. Hold it between thumb and finger and twist gently and the covering sheath should come off. After the sheath is removed the developing leaf will unroll rather than continue growing to full size inside the sheath. Obviously you need to remove the sheath before the bud reaches full size.
I really think all that is a few years into the future unless you plan on having a really small banyan fig bonsai. you would be much better to ask about strategies to increase trunk thickness and develop branches than smaller leaves at this stage.