See, they need to be a different size than the pattern is supposed to make. And normally that's basic math. Do a gauge swatch, do a little math, find out how many stitches are needed, and adjust pattern accordingly. It is not working out according to plan today. I have ripped out and restarted the mittens 4 times so far, and half the problem is that my gauge swatch was a big fat liar, giving me an extra stitch per inch. The other main problem was me missing the fact that they use a Norwegian construction where you start with less stitches on the palm side, and I was doing the calculations as if palm and back started with the same stitch count.
So the day went like this. Start with 80 or 90 stitches, realize it's too big a cuff, rip out. Start with 72 stitches of cuff and increase to 86. Realize the increases should be part of the thumb gusset instead and rip them out. Start the pattern, realize it's divided incorrectly between palm and back of hand, rip out. Restart the pattern, realize halfway up the thumb that the mitt looks big, recheck gauge, discover your stitch count will result in a mitten too big by at least an inch, and rip out yet again.
Sigh. That mitten had better work this time (knock wood, hopefully I've made all the mistakes now...). I will get to it right after I make a cup of tea.