Get something that covers as much of you back as possible. My back protector is a little short (i'm 6'5) and I found out the need for well fitted protector this summer, the hard way.
Over rotating on a kicker, i came down on the top of my back / bottom of neck. Luckily my protector was covering this area, saving my spine.
Unluckily my lower back was not completely covered. My ski came off, rolled under me, and i managed to open myself up rather nicely. I luckily missed anything important around there, but needed a heli and an operation to stitch everything back up.
I have a great photo to share, but it might be too messy for here.. hmm