32-34" waist ish 
you probably don’t want used ones. go to LBS and find some. seriously.
eww
All the proper ‘cycling trousers’ I’ve ever seen are either Lycra tights or weird baggy things with contrast bum and knee panels.
I use some Nike tracksuit trousers, they cost about £15 or £20 from Sportsworld. If you don’t mind looking like a chav, they’re great for riding in (with padded undershorts underneath). The lightweight ‘outdoor’ trousers that Crag Hoppers etc. make would be good too and probably look much better.
Anyway, it’s nearly summer now so you might as well just ride in shorts!