L.K.
When my son went to a toddler bed, I just used the small decorative pillow that came with his crib bedding. When we got him into a big boy bed at age 3, I got him a standard pillow that wasn't very fluffy. He continued to use the small decorative pillow. Now at age 4 he uses the big pillow. So I would just start small for an easy transition. You could always get a standard sized pillow, and just let her choose for herself.