I have an 8" wrist as well. It depends on what the watch is being worn for IMO. For a slightly dressy affair I'm ok letting the watch slip under the cuff, in which case I'll wear a 40 or 42mm watch. For more casual stuff I'll wear 45-47mm with no concerns. With pams the primary thing to keep in mind is that the bigger 47mm watches (like the subs, 372, etc) sit tall on the wrist as well. So the watch looks even bigger then it actually is. The 47mm rads sit much flatter. Personally I find the 47mm rads best for me which is why I wear my 514 the most. But I do like mixing it up and will rotate in my gmtII and my 359 during the week and then go with the 389 on Friday, etc.
Buy what looks good to you, in the end you can always sell on m2m with little/no loss.