Agree that that F5 and even F5s are too big for skinny wrists. They just don't sit right.
I seriously considered the F5s but it was more than I wanted to pay and it just looks a little awkward to me in the way the strap lugs stick out so far.
I think the 645 will actually be smaller and thinner than the 235. See the images below from DCRM.
235: 45 x 45 x 11.7 mm
I wonder if those 235 dimensions listed included the protrusion for optical heart-rate monitor.
The photos you posted do not show the FR235 for comparison.
Nope.