I do not have my AE to open from this computer. However I think it is under here. Settings. You will first set your end point via cursor and then hitting "N" to set the end mark of your timeline. You might even be able to ( right click in TL to access Trim Comp to work area). Sorry for not giving the exact path. There again, I am on a diff puter.
I am not exactly sure what you are asking here. After Effects projects do not contain footage, they only point to your original footage. If you have a 1 hour video and you're using 10 seconds of the video in an AE project there is no way for After Effects to trim off the unused source footage. You'll have to cut down the original footage, render a cut copy, and use that in your AE project.
If that's not what you are asking try making the question more clear.
Rick I think he made his Comp to long and now wants the edited version or the remains. Hence, the idea of trimming work area. Sliding the start point to the start of the comp "as I assumed he would do" and the using "N" to set the eNd point. R.Click to access menu to trim comp to work area. I agree he needs to make his questions a bit clearer. Yet it was only months ago I was looking for the same answer...
I shudder to think how many times, when I started (and embarrassingly enough sometimes still) I rendered out a test clip, and wondered why it took so long - only to watch a great few second clip, with 4 minutes of black screen afterwards!
Yes "B" and "N" for your project workspace becomes your best friend!