hi if you have a 82" drop go for curtains with 90" drop - provided thats not going deeper than the floor. Just think how far you want them to hang down. Drop lengths are fairly fixed if you buy ready mades.
As regards width I assume you will want nice curvy pleats as opposed to a tight sheet of fabric so if it were me I'd go for curtains with a width of 60" each (on basis you get 2 per pack). So you wd buy or make curtains with drop of 90" and width of 60" or a bit more. You'll get 2 curtains per pack usually. Plain curtains are difficult these days. 'Futon Shop' has nice simple stuff tho' and not dear either. Of course
there is always John Lewis if you can afford it. see y'a