sir william

try 2 get the same size,trim it out int.&ext,chaulk the outside & after paintin the trim & doin whatever ,silicone the inside around the trim.As for the r.o.[rough opening]you may get a smaller window & pad it out,but, you cant do a replacement wider than origanal, cause then it would probaly require to remove studs around the opening[aka liners& possibly even kings that are holdin up the header.good luck,& thats just 1 mans opinion & 90% fact.