dimensions that do not require you to do any cutting of the above rafters.
Most ceiling and attic flooring joists are 24 OC. OC means on center. So, the center of one rafter is 24 inches from the center of the next, BUT, they are about 23 and some change apart, if your with me. Edge to edge.
Now, you want a drop down ceiling stairs that fits in the dimensions of a 24 OC joists program. If its alittle too short for the rafters, you can always shim in and allow for this.
IMPORTANT;;;;; you do not want to cut any rafters. this will compromise the building skelton and could cause problems.
Lenth is not an issue, you just need to allow for walls, electrical lines, and any other stuff up there that cannot be moved.
Hope that this helps. The bottom line is that you need to see the spacing of your rafters. Then when you go to the home center, mention your rafter spacing and then buy accordingly.
Putting one up is definately a two person project, unless you build cleats to hold one end while the other is fitted in.
Be sure to measure the ceiling height because they come for the standard ceiling height and for 10 footers.
Then you will have to cut the angles on the legs when you get it installed. this is easy, you want the same angle as the floor to step.