Incidentally, is it possible to make the trash icon larger in the desired location inside the Opus Explorer?
Not without increasing the size of everything. Items in the folder tree are all the same height as each other. You can change the font size (for all items), but the icon size is defined by Windows (16x16 multiplied by your DPI scaling factor).
Can I put the trash in the folder tree?
As in the Recycle Bin folder?
It's in the tree by default. If it's not in the tree, why were you asking for the icon to be larger? I'm confused.