You will know the hide is too small because the snake will wear the hide as a hat or turtle shell.
My large Blood used cat litter boxes until she started wearing them as a hat so now she just has extra crumpled newspaper.
When you get new hides they will probably be a bit large at first. An easy fix to make them feel smaller and more secure is add moss to them. Put damp moss in during a shed and you'll help their sheds as well.