When should i upsize her crate

I have a 3 month old gsd puppy, she is in a smaller crate rn it has about 20-30cm extra room when she is laying down and when she sits she has to put her head down a little but i dont know when i should swap it out for her new "adult" crate. The new one is much bigger and i think it would be a little too big for her rn. so when should i make the switch and how can i make it less stressful?