Take the yarn and needles you want to use, figure out how many sts per inch you get, multiply that by how many sts you need.
so....gauge swatch...to get sts per inch...got it.
Then see if one of the larger sizes has something close to that many stitches (front and back combined) and use the instructions for that size.
GREAT idea!!! (but probably not)??? so what then....
For the rows, you'll have to convert their rows to inches and knit by measurement for lengths. So if it says to knit 12 rows in one color, you may find that's about 2" and knit that many rows in your gauge to 2".
Should I make a mock up for measurements?