Eyup chaps, now this may sound stoopid, (leavin misen wide open here), but what size is a 15-20 litre tank bag? I know you can get different sizes, but litres are a fluid measurement, and we don't carry oggin in 'em, do we?
I got a 25ltr tank bag that expands to 30 ltrs, it will hold 7 tee shirts & pants and a pair of trainers, 1 pair of jeans and 7 pairs of socks if jammed in the side pockets & a small wash bag, so have to leave me makeup & frock behind when away on a trip!