I'd say wait a while to see how they play out.

Small fruit size may not persist, as you say, into the generations going forward. Seems to me the fruit size may be of importance mainly for processing needs, bigger might be better in those instance, less handling for jam, tarts or wine. My direct experience is rather limited to fresh eating, (though I've tasted jam that was lovely.) Was the recipient of a large variety of the leftover medlar from the last couple of HOS All About Fruit Shows. Got to monitor the bletting  of at least a dozen varieties each of those last two years. We concentrated on collecting a larger sampling of medlar to show, those last. The point being that my considered opinion was, for fresh eating, there was a smaller fruited variety that presented as my favorite because of its exceptional flavor and texture, not dry or stringy. Perfect for enjoying and refreshment/replenishment in the van after or between jobs, no spoon required.  Superb. I read John S secured scion wood of that one called Ka Kheti. Was said from wild (in that case it'd be selected from a seedling surely,) from Georgia, on the another continent. John may pipe in to affirm the observation or not.  Anyway, small is not necessarily a deficit. Ka Kheti was the smallest fruited of all the collected samples and if memory serves most of the varieties or far more, in the repository collection, were more mid size and larger, bigger and huge. 

And sometimes, it seems, it takes a tree or shrub some time to come into its own. I've a Pawpaw that consensus was, by those who had a lot of depth of experience at the time, really not that good tasting of a pawpaw and thus, had been and was no longer even offered in any catalogues to their knowledge. I thought seriously about using it to graft something more reputedly delicious on to. Nothing offered up, by way of scions at the time.  Seems these decades later it has now become more the favorite of my pair. For the last few years at least, we'll soon see about this year. Oh, and I had previously noted that variety was an exceptional pollination partner for my other tree, because of its earlier to bloom and extended flowering period.

So, for at least some of those 'reasons', I'd think to wait a bit to see what develops. I don't grow any medlar. Discussions have suggested they are not too fussy about grafting, a lot like apples I'd suspect. Maybe shorter scions to hang on to, (...from some past pruning efforts I'd seen.) Reportedly they even take well to Hawthorn roots/seedlings, (though it seems to me that might result in kind of a bigger bush.) As you've noted, you can still graft on at any point you determine.

Yep, Pretty small fruit. Nearly on par with the ones previously mentioned at Johns place, though his Ka Kheti is a bit overstoried by the house and taller plants thus more sparse, his fruits also are a bit smaller than what I recalled. The pickings for the event I remember happened just prior to the third Saturday in October, from the repository in Corvallis so maybe with the rains onset there may also be a bit of sizing up? Nah.  I figure your foliage is lush so they appear to be pretty well exposed to the sun. Karen T, when asked for her recommendations of medlar, longer ago, suggested bigger was better. That the taste variations were not so important in the processing. Nottingham I think has been put forward and the arboretum now HOEC, had also Monstruese (sp?) you're on your own for the spelling, sorry, I figured it meant monstrous in size as is my habit to make joke inference, but it's French, so something else. Was quite large, bigger than Nottingham. 

It's a shame, truly, that you won't be able to establish a relationship of interest with support staff at the germ plasm repository for a time. Due to the Gov't shutdown, those workers will be furloughed or worse, the land block with the Medlar collection will be inaccessible. It would be worth while getting in there to see that array if possible. and you are local there so you could pester them. Maybe go tomorrow with fingers crossed they're slack in the shutting down and need some help with the gate.