So, the length of the exported clip depends on the selection before exporting. If the tracks are imported as clips of various lengths, selecting the clips before exporting them should set your selected range identically to the length of the clip.
Then, when you export, the length should be the same as the clips your were starting with.
If your clips are all the same length, but have silence at the beginning or the ends, then you will have to find another way to select the correct time range. One method would be to use markers.
I use File > Export > Audio when exporting to Wave files.