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 object mode, move the pointer near the beginning or end of the region. The cursor will change to indicate that trimming is possible, and you then Left-drag the edge of the region.
Trimming will obey Snap settings.
Left-click in the colored bar at the bottom of a region. If you are nearer to the start of a region, this will trim the start time to the position of the pointer. If you are nearer to the end of a region, it will trim the end time.
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).
Sometimes you just want to extend the start or end of region so that it reaches the end or start of an adjacent region. There is now an operation accessible from the region context menu, under Edit >Trim > Trim to Next or Edit > Trim > Trim to Previous. This will extend the selected regions so they directly adjoin their neighbours, unless their source files are not long enough, in which case they will be extended to the maximum possible. Trim to Next will extend the end of the selected regions to the start of the next region; Trim to Previous will extend the start of the selected regions to the end of the previous region.
These are not bound to any keys by default, but could be via the Keybindings Editor. They can also be sent via OSC or other control protocols.