Not sure if it will be too small or not... but you could try getting a size shorter belt to maybe get a bit more tension on it if the tensioner bolt is maxed out. You definitely don't want to over-tighten the belt though. A can of belt dressing is a band-aid fix, but it will temporarily relieve the squealing. I actually made a bracket to mount an idler pulley between the alternator and crank to put more belt wrap on the alternator pulley on my car. This was the only way I could fully eliminate the belt squealing on my wrx in cold weather.