You can use a new tool in the build menu to measure a few different things:

Room Volume Measurement Tool: Enable this tool and hover over a room and a blue box will appear inside the whole room along with the volume in the room at the corner of the screen. You can also see what % of the ship the room is

Length Measurement Tool: Enable this tool and drag across your ship to see the length of it.

Gun Measurement Tool: Measure the recoil & accuracy of your gun