This is one thing that bugs me about Inkscape! Suppose i have a line that's 3 units high, 4 units wide and therefore the ACTUAL line length is 5 units (as this forms a 3, 4, 5 triangle). If I now want to change the length of the line to it's half (2.5), I can't just change the height or width as it doesn't maintain the angle of the line. There appears to be no easy way to change the actual length of a line and keep the angle it's at the same.

I'm sure there is a way to get to the properties of an object and that for a line it's possible to edit the actual length whilst keeping the angle it's at the same. I just can't remember how!

The "Transform" panel will let you scale maintaining aspect ratio by percentage for instance if it neeeds precisely or hold ctrl-key down while click&drag from one corner.