What do you think? Would a 64" door be too short, and allow water to splash
over the top of the door?

The shorter doors are the common height for installing on tub-shower
combinations. The taller models are intended for
shower-only with floor pans. If you don't thrash around a lot when you shower
go for either one. If the shower head is visible over the top of the short one
better go for the taller model. HTH

Joe