But then on the other side of the coin...
Once you are in 16-bit, whichever color-space you save the file won't affect file size at all. If you want to be totally progressive and anal about efficiency in your color space choice, then I suggest you invest in Joe Holmes' D-Cam 4 and D-Cam 5 sets. If you really want to stay in 8-bit, then look into his D-Cam 3 space.
The issue here is having a color space that holds 100% of your capture device's colors -- even if output devices get to the point they can exceed the capture device's space, it is moot since all you can ever show is what you captured.
The real advantage of Prophoto is it is so large, it is unlikely any capture OR output device will exceed its size anytime soon... The disadvantage is it is a gamma 1.8 space, not a gamma 2.2, so many image editors mess it up with contrast or saturation changes. Here is where Joe's saturation profiles help immensely as they're based on the proper gamma. This brings me back to Joe's D-Cam 5 space as probably the ideal, but it is not an accepted standard like Prophoto...
PS: FWIW, the same thing was said of the Adobe RGB space about 4 years and longer ago, that it was large enough for real-world devices... And right now, we've got cameras and printers exceeding Adobe RGB in some areas. As Yogi Berra said, "Pre-planning must be done in advance!"