I feel like, at least for me, having astromechs of varying sizes isn't that horrible a thing. I know that most, if not all, of the astromechs we've seen on screen were roughly the same size, but chances are there'd be at least some out there a little bigger or smaller than the others. Like Hondo's droid they released a while back- I kinda like the idea that Hondo saw this little runt of an astromech and took it in to make this crazy death machine. Same with Aphra- she took an astromech that would probably be made fun of for its size, or an outdated one that nobody cared about, and made it into a one-droid army. I don't think I have it any more, but that First Order R5 droid too- it makes sense in my head canon that the First Order would cut costs wherever they could and go for the cheaper droids. The more important ones like R2- yeah, it's nice to have a correctly sized one, but I don't mind if one or two are smaller.
Speaking of First Order droids, I'm surprised we never got BB-9E in the Black Series. I feel like such a big deal was made out of the BB units back in the day.