I have retourne Kellys 28 in Box and Togo. I use them for everyday and also for plane travel. They are plenty big for me as I don't carry much each day.
I also have retourne 32 in Box and Togo, and I like them for a change in look, but they are heavier and take up more physical space.
I only have one sellier Kelly, in a 32, and I really love the way it looks. I don't like sellier for everyday. I find it hard to put my hand inside to get at my things. For this reason I would never consider a Kelly sellier in 28, which would be even smaller. I find that my black Box Kelly 28 is dressy enough for day to night even in retourne.
For me, a "small" Kelly is the 25, which I also love. I have one in retourne togo and it's an even better day to night bag than my 28.
The Kelly always looks dressy and polished to me no matter what style, size or leather.
But I find I can always dress it down easily. This versatility is why I love the Kelly so much.