I can speak from experience here.
I'm 6'5" and 300lbs.
My GF is 6'0" and less than half my weight.
I too have a Ford van, an E-150.
We're not living in it full-time right now, but are camping in it several times a month.
My original plan called for a bunk at the back, crossways with her feet in the driver's side corner.
The interior body dimensions at the belt line is 75" wide. Take away an inch or so on each side for foam insulation and paneling, put the space right about 72" or 6'. Basically a twin bunk for her, but a couple of inches shorter.
Less than ideal, but being her own bunk she would have had room to sleep a little diagonally and been fine.
My bunk was going to run down the driver's side wall forming an "L" with her bunk, our feet towards the corner.
My bunk would have been about 80" long, so basically a twin XL for me.
Both bunks would be fixed in position with storage beneath my bunk and nothing under hers, so van could still be loaded with things from the rear.
The 36" bunks would meet in the corner giving us both a large area for shared leg room.
All layouts are compromises but this one allowed both of us to both swing our legs over our bunks for ease of entry and without disturbing the other getting in and out. With the beds made and some pillows against the wall/windows, we had an "L" shaped seating area.
Like I said that WAS my original plan...
Instead she wanted to be able to sleep together and snuggle.
We ended up putting in one sleeping platform crossways in the van.
It is way too short for me, even sleeping at a slight diagonal.
I'm claustrophobic so have to sleep along the front edge, she sleeps at the back and feels closed in.
Long story (sleepers?), short we're miserable.
So now we have a large almost-queen-size-but not long-enough sleeping area that takes up the whole back of the van, permanently affixed and we're not happy with the quality of sleep. We're examining alternatives, including going to my original design. Something definitely has to change.
I'd urge you to go with a twin XL for you and a twin for her.
Whether you make it fold up or it is in fixed position, you'll both be happier with your own sleeping space.
The other alternative is a queen-sized long ways but be prepared to lose a ton of space.
There ARE ways to get a queen-sized, full length bed in an E-150/350 yet retain some floor space by making it disassemble.
But the reality is you probably don't want to be assembling/disassembling things and doing "mattress origami" to make and unmake the bed.