Have you searched online? I don't use Power Tab myself, but a brief search indicates that if you're cutting and pasting measures (to repeat a riff for example), PT automatically generates bar lines. Otherwise there, apparently, is
Section -->Position Width
then
+ spreads the notes out more,
and
- squeezes them together.
etc.
I used to use TuxGuitar, which is a Linux-based version of Power Tab (which apparently only runs on Windows), but have long-since switched to
MuseScore, another open-source music editor, one that runs on both Windows and Unix-based systems. There you select a bar and use { to squeeze notes together and } to spread them out.