The standard size for rack mount equipment is 19" wide.  The height of 1 rack space is 1.75".

Here is a list of sizes for Standard Rack Cases:

1 U (one unit) - 1.75"
2 U - 3.5"
3 U - 5.25"
4 U - 7"
5 U - 8.75"
6 U - 10.5"
7 U - 12.25"
8 U - 14"
9 U - 15.75"
10 U - 17.5"
11 U - 19.25"
12 U - 21"

Be sure to check your equipment to make sure you buy a large enough Seismic Audio rack case.