You should be able to slip two fingers under the collar comfortably, if not, loosen the collar at once.
It won’t be long until they need the next size, so why not get one in advance and put it in a safe place until it is needed.
When you switch collars, remember to switch the identity disk over too!
I have known occasions where an owner has neglected to do this and the collar had tightened to the point of needing to be cut off. Any tighter than this and the collar could embed itself in the dog’s neck and then it becomes a serious welfare issue.