I don't wear some of the pieces and some aren't something I'd buy. I will only answer for what I'd actually wear.
non-band rings- I think the largest I'd actually wear would be a 6 carat old mine or a similar size step cut. I have a 3.09 old mine and it is a nice size and noticeable, but double that is probably the most I would wear. Realistically, no more than 11 x 11ish max for the center at very largest. Sweet spot is probably a high 4.something to 5 realistically or 10.2 to 10.5ish mm size.
Studs- I don't wear these often. I have a pair of moissy OEC cuts and they are 2 ct equivalent. Moissy studs suit me fine honestly. I don't get to see them so I'd rather spend where I can see them.
non band rings- really depends. I have one ring that is late deco and I wear for travel. I love it. It has a .15 or so center in a scalloped frame design. I like it so much that I am having a replica made with an antique oval that is 1.4ish, I think?
Studs- probably .50 ctw would be the smallest for me. I am not a tiny person.
I think the size choices make sense. I like the use of the word “realistically” - sure, id love a 40ct diamond but what on earth would I do with it?? I love step cuts - they look so gorgeous in bigger sizes and somehow less overwhelming than rounds even though they cover more finger.