You can measure a segment with the Measurement tool - essentially a ruler - which is at the bottom of the toolbox. Of course, you can also access it using it's keyboard shortcut, the M key. Enable snapping, and snap to cusp nodes. Click and drag to measure the segment.
Divide the segment length you want to scale it to by the measured segment length, and multiply that by one hundred to calculate the percentage for the scale.
Using my example above, that would be
10/45.24×100 = 22.1043324%.
Then do a Scale Transform using that percentage - that should get you very close to the desired size.
Example, showing measurement after scaling (zoomed in a bit):
However, note that Inkscape is not designed for CAD drawings/plans - it's graphics software used for creating SVGs, not for engineering/architecture, and lacks the mathematical precision that CAD tools offer.