What are you generally wearing when you try on the rain jacket? I would generally advise sizing it a little big, since it generally will be the last layer you put on. For me that means if it's cold and raining (and I'm not generating much warmth by hiking) I have on a shirt, light fleece, down jacket, and then the rain jacket. My rain jacket can fit over all of that. Of course that means it feels a little weird and baggy when it's warm and I'm not wearing many layers, but it's better that way than needing to put it on when it's cold and having to decide between wearing my rain jacket or my insulation. Of course this can depend on the conditions you generally expect to be hiking in. If it's almost always warm when it rains where you hike it doesn't matter as much.
Not really addressing your question, but this same subforum has a thread ("Rain jackets, what do you use?") discussing various ways of dealing with the rain, ranging from expensive WPB rain jackets to umbrellas to ponchos. Just in case you were interested in considering other options.