This is reminder number three thousand seventy two that it’s better to set your thumb stitches aside on a piece of scrap yarn (rather than a spare needle) if at all possible.
Because you really do want to try your mitts on at about this point, just to make sure you’ve made enough room for your thumb. And the yarn (which curves! like how your knitted fabric curves) will do a much better job of letting you gauge the fit than a needle (which does not curve! which will, in fact, distort your fabric and feel like knives when you try it on).
Now to be fair, I sometimes only manage to do this for the first of a pair and then when I make the second one, I don’t bother, because at least in theory I already know how I want the thumb.
But it really really really is worth it on the first mitt if you want it to check the fit as you go.