Sideways Mittens in Garter Stitch

I have an Mon Tricot magazine.  Part instructional book, part stitch dictionary, part pattern library.  There is a sideways knit mitten pattern that I always thought looked neat.  Megan is wanting to expand beyond dishcloths, but I didn't think she was ready for purling.  She's done increases and decreases though.  I thought if I wrote the pattern out in a better format, she could follow it.  However, the pattern just says DK wool, needle 3.25mm, fits size 7 1/2.  Meg has big hands, but I don't know what 7 1/2 equals, and with no gauge given, it's impossible to figure the sizing.  I asked for pattern testers on a couple sites on Facebook, but got no response.  I figured we'd just wing it and see what happens.

Well, they're obviously too long and were getting too wide so we eliminated a bunch of rows for the width.  Then the thumb placement--done by short rows--was not right.  Tried to fix that, but the thumb is still too low and long. I'm re-writing the pattern to fit her properly, now that I have a gauge to work with (and she got 19st/22 rows in garter stitch.  It couldn't be 20st, could it?  LOL). I'm not sure how to adjust the thumb gusset.  The short rows went by 4 stitches, I'm thinking of doing three stitches, or even two.  But I just don't have time to test it out.  I feel bad for her, that I'm so busy and it's hard to find a pattern for something nice, usable, and easy to follow.  I know there's math--how many rows needed to go around the thumb, and how many stitches difference between the start of the thumb gusset and the longest part...but there are times that knitting math lies.  I searched Ravelry, but there doesn't seem to be an equivalent pattern.  We're going to rip this mitten back to the start of the thumb and hopefully get it placed right.  There was a bit of shaping issue with the top of the hand, it didn't come out symmetrical, but maybe it'll block out LOL.  Stay tuned for updates.  I also think DK yarn for mittens, in Ontario, is silly.  This would go a lot quicker if it were worsted weight, and be warmer.  I see the need for a few different mitten patterns :)