Basically, there are only 3 ways to make small leaves.
- One is to keep the tree very small, that is as a mame/mini bonsai, which clearly isn't what you are wanting to do.
- Second is to cut the compound leaves back to just 2 leaflets, 4 leaflets, etc., and by then removing one more leaflet, you can make odd numbers
- Lastly, you can defoliate. The new leaves are small and over time may enlarge to full size. Were you do defoliate in August, say, they may not attain normal full size before leaf drop. This, however, seriously weakens the tree.
It is also possible that the leaf size will reduce somewhat in bonsai culture. Many species leaves progressively 'shrink' over the seasons with roots confined in a small pot (what gardeners call 'root bound').
In brief, do #2.
Ed. in response to questions about what #2 means:
This whole thing is one leaf, the compound leaf of ash. I've marked red lines where you may cut it to illustrate.
Since the normal leaf has one leaflet at the end, cut leaves may look 'unnatural', which you can correct to a certain extent by also removing one of the leaflets on either side of the cut.