I've been looking for this option for a while but still haven´t found an answer...UI is too small...Is there a way to increase it?
You can't. this happens automatically since CC2014.1 (or supposed to). read here about Refined high-resolution (high-DPI) user interface suggestions: What's New in After Effects CC 2014
you can share a screenshot so we can see what issues you are facing.
if you feel there should be some new feature in Ae regarding this, you can submit it here and make Ae better for all of us: Feature Request/Bug Report Form