A standard Birch backboard is generally 18" x 72". Occasionally, you will come across one that is 20" - 24" wide and 74" long for larger/taller people. They are 1.5" thick because they have two .75" thick hardwood runners on the bottom to help with sliding, and to allow an EMT to get their fingers under the handholds. The newer plastic and aluminum ones are thicker due the materials they are made out of.
Originally we made our own wooden ones, and we made them 16" wide so we could get three out of one sheet of plywood.
The wooden backboards can be used to dig sand with (especially the tapered ones), as traction mats in an emergency, and as tables also. I'm thinking they would make a pretty good stationary 'creeper' to lay on to slide under your vehicle if you had to also. I wouldn't use one as a jack pad or traction mat unless it was an absolute emergency, because the load could fracture the wood fibers and weaken the board for use with a patient. Again, you don't want to be THAT guy! If you had to use it for that, I would take it out of service for medical use and replace it.