I only have resin bjds, except for Emmilinda, who is a Bradley doll. I try to limit myself to bjds because of how customisable they are and how they feel. I used to want some vintage Barbies and older play dolls, but after seeing them at the local doll show, I realised how limited I would be with them. I like to make my dolls into characters, or find a doll to be an existing character. While Emmilinda, even with her fixed position, is a part of my photostories, it's far easier to play with BJDs.
As for size, my dolls range from 25cm to 43cm. I'm still hesitant to go larger than a mini as for many years, I just felt that that was too big for me. One of my good friends has a number of SDs, so I play hers. I'm slowly warming up to the size, but I love how I can sneak the tinies into my bag. The minis are great for making clothes for and they're still easy enough to lug around with me.
Anything smaller than Finnian, my 27cm but skinny boy, is simply too small for me. Stringing and repainting him is a pain. I'll gush over other people's little tinies but knowing me, I'd lose the doll somehow. That actually happened with Fleur's little doll. I went to a doll show and the little doll, a Hasbro Dolly Darling, fell out of my pocket. I searched the floors and told the doll club, but she was never found.