G'day Magenb! Happy Fathers Day!!
Your Terrain is coming along nicely, mate. Your sculpting skills are matching your painting.
To throw in my two cents worth on the size of a piece, size doesn't matter as long as it has a purpose. The largest pieces of terrain I have will block line of sight, but also can be fought upon/amongst, (Litter the blessed thing with stairs).
The cheapest place to collect clear plastic sheeting is your local sign writing business. They will have truck loads of off cuts, from their light box signs, that they'll be throwing out anyway.
Luke Towan has a Utube channel that shows you how to build truly realistic trees on a budget. Hope any of that helps, Magenb. This table is going to be amazing when you've finished... Be well!