Hairy hielanders didnae wear silk, but I suppose your mother isn't going to go for the 2-yard wide, 6-yard long plaid of the pre-proscription era!
I've made sashes and if the fabric is woven rather than printed then all it requires is a rolled hem (hand is best but machine will do). If you can't stretch to that or the fabric is printed, do it double thickness. The overall length will need to be whatever dimensions the body wearing it is, eg, from calf up to shoulder and down to shin plus allowance for tie-off, bow or whatever. The finer the silk the more width of fabric you can use without it looking bulky. Personally, I think more is better within the limits of trip hazards. Too small an item and you're in Bay City Rollers territory!