If you want to see/test out, firsthand, how much various size fenders or collars would add, then take a few swimming with you, & see if you like the results when you try & depress them.
And, of course, you can simply calculate how much bouyancy you'll get for fender size X. As the math is that for the volume of a cylinder.
Another option might be, to find some inexpensive foam which doesn't absorb water
. Be it solid foam, the pour type, or some of both. And then sew your own collars/covers. With or without an opening flap with double velcro closures, much like the cuff on a surfboard leash. So that installing the foam inside of the collar is a no brainer.
Just use some good UV resistant thread, & a DIY grommet kit. Or use stitch on webbing loops, etc., to use to attach the collar to your dinghy with.
Also, were it me, I'd paint the inside of the collar everywhere which wouldn't get sewn. So as to provide the foam with more UV protection.
Plus, it's an educated guess, but if you went this route
, & used some recycled materials for the project, then this might be a cheaper than fenders. For example, were you used an old sail that's lost
it's shape, but has lots of life let in the cloth for the collar portion, etc.
And you could use anything cheap, that's both bouyant & long term water tight, inside of the collars for added bouyancy.
I'll be curious to hear about the results though. As doing this to a dinghy has run through my mind more than a few times.