I think the rule of thumb is that the bottom of the Argyll jacket should end somewhere around the fell line of the kilt. (edit: Dang. Sydnie beat me to it!)

Going by arm length can be tricky, as this seems to vary. I usually see the sleeves about 1" to 2" longer than the jacket body. For example, this is a jacket I just won on eBay, and it fits me PERFECTLY. I'll be posting photos of me wearing it soon, but take a look at the sleeve length in relation to the bottom of the jacket. I'm guessing there's at least a 2" difference.