Dot pitch is the number of lighting spots (dots) per inch. The more dots, the sharper the picture. A decent monitor (CRT that is) has about 25-30 dpi.
With diagonal they mean the distance between two opposing (diagonal or cross-wise) corners. A 19" CRT has a distance of 19" between those 2 corners, but since part of the picture-tube is hidden behind the bezel (or frame) you will only have about 18" visible, also expressed as 18" VIS.
A 17" will have just under 16" VIS.
So, bigger is better.
If you want a CRT, look for a flat screen, SONY and some EIZO/NANAO use Trinitron tubes (which in my personal opinion are the best). Misubishi tubes are called Flatron and are almost identical.
There are only a few CRT-tube manufacturers, that's why some screens from different brands look almost identical.
In a 19" look for a possible resolution of 1600x1200 (if you are a gamer) and a refreshing rate of at least 85x per second.
The monitor is your most important PC equipment, so don't skimp on the price here. Your eyes will thank you for it.