I don’t have a rule of thumb as absolute size changes from small trees to big trees. It’s one of the main trade-offs in bonsai development - how to get the branch big enough to thicken the trunk but not too big that the resulting wound is hard to heal.
After removing larger sacrifice branches, I always identify another branch to help with the healing. The process seems perpetual up to a point at which the wounds are small enough that they close easily.
I’d guess this approach will work well with your elms, alders and birches - will look forward to seeing the results!