I also have a 15" Macbook Pro and I invested in a Wacom Intuos 3 6x8 tablet. I think it's the perfect size. The smaller ones are harder to work with (for me, anyway), and the bigger ones are just too cumbersome, especially if you travel a lot, which I do.
I wouldn't consider the 15" screen a widescreen, but as far as tablet dimensions being best suitable for this sized laptop, I honestly wouldn't worry too much about it. After you load the software for the tablet, open up the settings (under System Preferences>Wacom Tablet) to customize them to your liking. There will be an option under Tool>Grip Pen>Mapping where you can "Force Proportions." Selecting this option will tell the tablet to comform itself to your moniter screen dimensions and proportions!