They are close to the same size...although reaper tends to go a tad smaller than the somtimes cartoonish porportions of GW. But as the other poster stated, the scales are so close that a reaper fig is not gonna look that outta place....
If you plan on doing any tournaments however I would reccomend getting an extra GW fig for anything you are going to represent with a reaper fig. GW doesen't like it when people use other companies figs in their sanctioned events.
Cheers,
Wilhelm