I started off with only 60cm. I experimented and got a MSD size and just could not bound with her. It was weird. But then I have smaller dolls like Alice Chery Blossom ( puki size I think) and mini pullips.
Tiny dolls are great for quick photo opps, taking everywhere with you. 60 cm's are good for when you have some time to spend doing your photo opps. I love my big dolls though and I hope you do to.