It really depends on how you're put together. I'm 5'4" and a regular plate fits me fine, but a small is only a smidge small. I have an occasional dive buddy who is truly tiny (I would guess 5 feet and 90 lbs) and I was SURE she would need a small plate, but when I put it on her, it wasn't right. She was better in a medium.
Tobin's question is getting at the fact that, with a single tank, you have a lot of latitude about tank placement with respect to the plate. You can move the tank up or down to get your balance right and the valve where you can reach it. So if the plate needs to sit a little low to get the waist straps right, it doesn't matter.
The same thing can be awful with doubles, because you only have a finite set of places you can set the tanks, so the top of the plate really HAS to be at about T2. Then, if the plate is too short, the waistband will be on the bottom of your ribcage and uncomfortable.
Tobin's question is getting at the fact that, with a single tank, you have a lot of latitude about tank placement with respect to the plate. You can move the tank up or down to get your balance right and the valve where you can reach it. So if the plate needs to sit a little low to get the waist straps right, it doesn't matter.
The same thing can be awful with doubles, because you only have a finite set of places you can set the tanks, so the top of the plate really HAS to be at about T2. Then, if the plate is too short, the waistband will be on the bottom of your ribcage and uncomfortable.