Mittens are a tough call.. If you're an experienced knitter adding stitches may work, but the easiest way is to increase needle size.
That's what the gauge measurements are for on patterns. To test the yarn you're going to use and needle size that works for you. You want mittens to be warm though so you want a little tighter gauge.
Do a swatch with larger needles and see if you like the fabric. And I suggest trying to knit looser. Don't pull the yarn so tight when you knit.