While at Pennsic, I finished my fingerless mittens from this pattern. While not period, they are WARM and I loved them.
The pattern also calls for two rows of 8 knit between the increases for the thumb gusset (rather than going from 6,8,10 there is a second 8 in there). I ignored that and went with only one row of 8.
These mittens were a dream to have on at night when I went to bed. It gets cold at Pennsic and I slept with these on. The pattern is BIG but not unreasonably so. I'd say if you want a close fitting mitt, go down two needles sizes. If you want one that is a bit loose, go down one. I could easily fit my entire hand through the finger opening without any issues.
I am washing these to see if they will shrink a bit. I'll try the pattern again some day with a smaller needle - they are simple but fun. I'm not good with the complicated patterns and this was very easy for me to understand and knit up fairly quickly.