use a colour rating system that AFAIK, is standard across the industry.... For 19mm OD gas springs....
Green - 50 lb. initial, 65 lb. final - 640 psi
Blue - 100 lb. initial, 130 lb. final - 1280 psi
Red - 150 lb. initial, 195 lb. final - 1920 psi
Yellow - 200 lb. initial, 260 lb. final - 2560 psi
Black - U-pick the pressure between 500 - 2560 psi
Strokes of interest are 63mm, 80mm, 100mm, and 125mm....
The 25mm OD gas springs use the same pressures for each colour code.... but the larger diameter results in higher forces....
Green - 112 lb. / 153 lb.
Blue - 225 lb. / 305 lb.
Red - 337 lb. / 459 lb.
Yellow - 450 lb. / 612 lb.
Black - adjustable as above
Strokes of interest are 80mm, 100mm, 125mm, and 178mm....
Here is a link to their Mini Series....
http://www.n-forcer.com/content/pdf/Mini.pdf
There will be a small variation between manufacturers, but the basic idea is that if you purchase a replacement of the same diameter, stroke, and colour rating they are in theory interchangeable....
In most airgun applications, Blue springs are a bit weaker than stock and produce a smooth shooter.... Red are similar to stock or a bit stronger.... Yellow are too strong for most airgun use.... The 19mm OD is the common size to fit inside an airgun piston.... and generally needs shims/collars to center it....
Bob