A small window on the side with stats on the ship your building, like length and width etc, strikes me as more useful for knowing ship size. any kind of ruler would be hard to use to estimate precise length due to perspective.
The compass, or some other way to determine what will be the front of your ship, does strike me as useful in case you want to build ships without a obvious front or are into weird backwards flying ships that defy the laws of nature. I'd personally go with 000, 090, 180, 270 degrees instead of north or front.
A grid and a axis system is probably also some of the gizmos they'll put in. My personal suggestion to add would be a tape measure, that lets you put little flags on your design and tells you the total length of the route you map out. (think google maps) In case you want to measure some arc or circumference or something, or if you want to know just how big that cockpit window really is.
and ofc, put in a toggle between metric and imperial units.