Thanks for question. Let me explain a bit:
All Transform values are applied as a post-effect.
If You are in Custom Edit mode
and Your Random Transforms
are enabled, then Reset Size button
You need to "bake" transforms
to items and disable Random Transforms. This can be done with one button click "Store Random"
Then Reset Size Button will work as expected- reset Items size to original geometry size:
To reset rotation as well You can run this script:
for each in ($.trees.getSelected()) do $.trees.setRotation each [0,0,0]
Hope this explains how FP works a bit and You can find a solution in Your workflow.
Have a great week.