How to determine width of the nozzle, that is already installed on the printer?
I would find some wires (piano wire, CAT5 copper, paperclip, nozzle unclogger, etc. ) and choose one that should fit the nozzle (eg. a 0.5 mm copper wire will fit a 0.8 mm nozzle) and push them into the nozzle. Heat them up a bit to soften the filament in the nozzle and brush with a metal wire brush to easily clean it up.