Sliding windows come in two types, wrap around rubber and glue-in, in most cases aluminum windows are the wrap around rubber type, but there are still a lot of aluminum windows out there that are glue-in, almost all vinyl windows are the glue-in type, With the wrap around rubber type, both the slider and the solid sections usually measure the same way, but the solid sections can sometimes be larger to make up for being further out, the rubber wraps around the glass and then you pound the metal on the rubber, the rule for measuring this type of window is this:
You measure the daylight opening (in between the frame) then measure the depth of the rubber times two,
if the daylight measurement is say 27 1/2 "and the depth of the rubber is 1/4"
then 1/4" times 2 = 1/2" plus 27 1/2", "the answer is 28" and so on.
Glue in windows you simply measure where the glass lays, ( hint ) rather than having to guess the new glass size, before you go and break out all the old broken glass, take out the snap in bead and very carefully and measure the old glass, if the glass just has a whole in it, just tape it up with packing tape and bring it in to the shop, if all you have is little pieces then you should be able to measure the three dementions on the fly.
it usually measures about an 1/8th to 3/16ths of an inch gap inside the frame all the way around, again with the snap-in bead, the size has to be darn close, especially with the aluminum windows, the snap-in bead has to be right on the money otherwise it wont stay in, with vinyle windows just make sure that you have a gap between the frame and the glass and stay away from using seal tape, even most of the pros. don't use that stuff on location cause if you make a booboo then the chances of undoing it from the glass are pretty bad and you will usually end up breaking the glass, use silicone.