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.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.