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
