When you go wide screen, they usually cut the horizontal down some.
They don't cut the verticle dimensions. The screen size is based on the length of a diagonal line across the screen. That said, a 20" or 21" widescreen would be about the same height as a 17" standard screen. 23" widescreen is about comparable in height to a 19" standard screen and a 26" widescreen is about the same as a 21" standard screen.
I wouldn't worry about it though. For most uses, I would personally prefer a widescreen to a standard screen of the same quoted size. The width is beneficial for multitasking as well as giving you a wider view angle for games.