It's true. The 13, 15, and now discontinued 17" models all had exactly the same size keyboard. The only difference was the amount of bezel along either side of the keyboard. The keys are all exactly the same size, same spacing, etc.
Now, whether or not that extra room on either side would be enough to help you I can't say, and only you can decide if it's worth the better part of $2K to find out. Generally if the issue is related to posture, the keyboard size isn't really the problem.
I have been using a 13in MBP for more than one and a half years now. Since my work involves a lot of typing during this time I used to type away for at times several hours on the machine.
However, after the first six months or so, my left wrist started to hurt. I realized that this had to do with the posture I was adopting because of the relatively cramped keyboard (I am 6 ft and have big hands).
My temporary solution was to get an external bluetooth keyboard and monitor. Since then things are somewhat better.
I have generally been wondering since then as to whether it makes sense to shift over to the MBP 15in? While I have not had a chance to measure the length of its keyboard, I assume that it is somewhat bigger than what Apple provide on the 13incher?
I did ask a friend who said that he had seen a 15in MBP but it has the same keyboard size as the 13in!
So, I am unsure if shifting over to a 15in MBP simply for the ease of typing makes sense. Can someone shed light on this?