It's about 1 -2 cm , this is an image of it about a week before it closed off its hideYup, very normal. I would keep whatever you keep water in close by and filled every few days or so, in case it is thirsty.
How big is your sling?
What did it look like before it closed off its hide?
It was hard for me to tell if it was in pre molt, the only reason I even have this picture is because that was its first meal in 2 monthsHard to tell if it is in premolt at least for me. Its abdomen looks good to begin with and this mealworm would have filled it out. It may be waiting to molt in a secluded place. Still put water near the hide. Read a book. War and Peace, for example.