Changing the length of a region is a very common editing operation, often known as trimming. There are several ways to accomplish this with Ardour, and some very useful specialized trimming operations.
In Grab mode, moving the pointer near the beginning or end of the region changes the cursor to indicate that trimming is possible, and the edge of the region can then be Left-dragged in both directions.
Trimming will obey Snap settings.
There are several commands for region trimming. Some use the edit point to determine where to trim to. Some are not bound to any keys by default (but could be via the Keybindings Editor).
These command are both in the Region > Trim main menu (with a region selected) or in the contect menu of a region, right click on a region > Name_Of_The_Region > Trim