Weird the line tool isn’t working - i guess you’ll have to troubleshoot that if you really want to, but to be honest i didnt even know the thing existed until I saw people asking about it.
The grid and snapping is your best friend.
The grid is automated, it changes scale based on how far you zoom in or out. It goes by metric system, so it just goes from cm to m to km, etc.
You can easily get some size references by dragging in a cube from the place actors panel. Default cube is 1m. If you scale it by 10, then it’s 10 meters, and so on.
If you don’t like popping in and out of ortho views, you can just set the scale of a few cubes - 10 meters long, 5 meters tall, 200 meters wide, etc etc. Move those around to use as your references.
Or drag in default mannequin for human reference. It is just about 2m tall.
With the translation snapping it also goes by metric. Just drag in your 1m cube and try out the different snapping settings and you’ll see what they refer to.
The default unit of measurement in unreal is centimeters. So if you set a characters max walk speed in movement component for instance to 300, that is 300cm per hour. To convert to km/h you just google the conversion rate, etc.