Hello and welcome.
Wise move,joining this bunch.
My limited advice would be to try different sizes at the show, go round local dealers, at busy weekend times, not only to look inside vans, but to talk to their customers,parked there. The more you enquire, the more you'll learn.
Big vans are ok, but do have limitations as to where you can go with them.
Are you going to use your van for long terming ?
Would you prefer mainly campsites, or wildcamping? Do you really need 4 berths? A fixed bed is often a waste of valuable space.
There's always a trade off and be prepared to compromise.
We spend half the year in our 6.2 metre van. We can get it just about anywhere a Tesco delivery van would be able to access.
Have fun and adventures.