I'm doing something stupid with dimensions and / or scale

I am drafting up an idea for a house extension.
I am using milimetres for units.
So the width of the house is 5740mm.
When I add dimensions, the defaults are really small so I can't see anything. See pictures.
Are my better off changing my units or dimensions or something else?

Set General Scale to the inverted value of the intended printing scale, for instance 100 if the intended scale is 1/100. Then the text size and other elements of the dimension will be enlarged accordingly.