Different wools felt different amounts -- so your best test is to knit a fixed size swatch, say 6 inches by 6 inches, and then to felt it down to a fabric you like, and measure that. This will give you the amount it shrinks, and you can apply that to decide how large to make your pre-felted mittens.
For felting knitting, I find a slightly open fabric felts much better than a tightly knit one. So I usually go up several needle sizes from the size recommended for the yarn, to give a good felt result.
If you just want to dive right in without sampling -- well, it's a risk, but we all do it! I'd say go up 3-4 needle sizes, be prepared to unknit the start if the fabric's still looking like "normal" knitted fabric, and keep a close eye on the felting to ensure they don't get too small. Aim for something at least 1" longer and bigger across than you'd want for a tight fit, possibly even more if she has large hands.
(posted by me this day on spindlers)