If I have a line that's at say 45 degrees and it's 10mm high, but I want it say 15mm high, just changing the the Height value also changes the angle. I seem to remember reading somewhere that you can access an objects properties and change the length rather than the height, and that this would maintain the angle and just increase it's length. So the question is: how do I access the properties and which one would I be wanting to change?

Thanks!

I misread your request. So, I am deleting my response. Sorry.

Suggestion. A different approach. Draw a rectangle that has your needed height and width. Draw a separate long vertical line.

Select the vertical line and select the Transform Panel. Go to the Rotation Tab and enter 45 degrees and then Apply..

Move your diagonal line that is set to 45 degrees over your rectangle. Then use Snapping or some other means to place the endpoint of the diagonal line  at the base of the rectangle.

You could draw afresh (re-create) your needed shape ... knowing that you have the needed height and needed angle available.

Or, you could do a Path Operation on your shapes, to create your needed object.

Maybe this will help.

First, turn on the node transformation handles.

Now this.....

Maybe this: