The OP said that the pistol is smaller than a G17 yet larger than a G19; based on that statement, I'm guessing we're talking about the full-size.
The size of the M&P9 FS roughly splits the difference between the G17 and G19, while the M&P9c roughly splits the difference between the G19 and G26.
"Smokey, this is not 'Nam. This is bowling. There are rules... MARK IT ZERO!!" - Walter Sobchak