....cut...... I've never owned a van so I have no idea how much room is back there. We have been using as truck/topper combination forever and it has worked for us ok. ....cut.....
Since you haven't owned a van before, here is my long 2 cents....
I think your idea to see and experience a low-roof van in person before deciding is a very good one.
Regarding size, so much depends on your needs and expectations. I presently have a low-roof Econoline (no raised roof added) and that's by far the reason I want to replace it. Therefore I'm admittedly biased. My van is getting old but in great shape, and if not for affordable new vans with "high roofs", I wouldn't be looking at all. Personally, I wouldn't consider a low roof Transit unless I had to park it in a garage and nothing else fit.
For me going from Class C motorhome to an extended van made it seem small. For you going from a topper to a LR Transit should feel more spacious. Even so you should probably compare side-by-side because to me even the mid-roof Transit seems too small. Particularly in width. So why settle for "ok" if something else might work even better?
If you are going to use it for any kind of camping, the bigger vans offer many advantages at a small incremental cost and not much compromise in maneurabilty. For me if it doesn't fit in a garage it might as well be as big as possible (except for Transit tall roof).
If you haven't seen a low-roof Transit at all, I suggest taking a good look from many angles. For my taste, most pictures on internet make it look better than it does in person, particularly in lighter colors. From some angles it looks truncated to me. In fairness the same happens to me with ProMaster and Sprinters -- low-roof vans look chopped compared to mid-roof models.
For what it's worth, I'm not questioning people who have purchased low-roof Transits in any way, just sharing my opinion that low roof models may not be for everyone for multiple reasons. Based purely on personal observation, it seems that like with ProMasters and Sprinters, taller roof models are more common.