If it's the axle bolt I THINK it will be a 15mm but I know its got those adjustable drop out things with allen bolts as well? A picture might help but if its the allen bolt that you have rounded, then you might have to drill it out and source a new one. If you're not comfortable with that sort of thing I would take it to a bike shop and let them have a look, It will probably only take them 5 minutes and be fairly cheap.
Good advice that


weather hasn't been great for MTB so it's been lots of road. Itching to get back onto the dirt but after so many storms there's still too many trees down in spots and too much mud.
