I tried a couple of heavy 18" field/hand cymbals a couple times (1950/2200 gm). Those were a bear to just move with your foot! Great chup....but not so great stick. It was a lot easier trying to move a pair of crash cymbals in weights of 1450/1700....but still a chore.
I sometimes pair my Paiste 18" Giant Beat multi up as a top with my vintage and heavier 18" Stambul 65 as the bottom. Lovely low and sloshy loose sound. I think that 18's should be played loosely and not tightly closed. They will probably then lack definition. But I hardly ever play my hats tightly closed anyway. I love this combo.
17 has been my limit. I like them, but they lack a chick sound. My 14 and 15 pairs are better for that. So it just depends what I’m doing. I used 17s in a gig yesterday, but I’ll admit, I really missed the chick sound that I wanted in certain songs. A trade off either way you go.