The max height for source entry fields is usually too small. I've found some cases where the box will expand, but I can't reproduce it reliably. Switching back and forth between VE and source will always result in a source field that's too small.
In the side rail, they're also too small, which means a tiny editing space.
Here's the editing field in the side rail for VE:
Switching to wikitext mode, the editing field is ridiculously small:
Regular post, the source field is too small:
Regular post, occasionally I'll see a source field that sizes appropriately:
I start to edit the description (header)
I switch to VE, switch back to the wikitext
Field is now a small height and there is not even scroll bars