Okay, would it be this one, or this one? Those are the only 2 that looked like long coats. I can't quite find the yarn used on the site, but it probably doesn't matter.
With the yarn you want to use, you can double it to see if you get the same gauge and that it's not too dense and tight a knit. If it is too tight, then use the yarn single stranded with the needles you want to use on it, and see how many sts per inch that makes. Multiply your sts times the finished measurement you want the coat to be at the bust, then see if one of the larger sizes is close to the same number of stitches you need. If so, use that size, but make the length the same measurement as the size you'd normally knit.