It will usually say inside the rim when you take it off how wide it is and the offset aswell, if you cant find can always just measure it up. In regards to offset basically if you can roughly work out how much further you want the rims you have now to sit out, then once you know the specs of current rims you can do some calculations or easier yet stick the info into a online offset calcualtor along with the size your looking at.
It will give you how much further they will sit out and protrude on the inside, from there you play aorund til you get around the figure you calculated in the start of how far you would like the your current rims to come out.