If you just need the size, put the cursor at the top left outermost point of your layout (plus some safety of at least 0.5mm) then hit [Space] and go to the bottom right outermost point plus safety… read the relative distances and you got your board size.
Drawing an outline works the same way.
I suggest a grid of 1mm or 0.5mm (or whatever that equals in inch) for drawing/measuring.
PS:
Here is another way to skin this cat though, working into the opposite direction … the fabs I use do charge 5x5 cm and multiples of those in each direction.
Most economical are 5x5 cm and 10x10 cm, so I try to collect a couple of layouts/projects and make them fit into that (saves money and allows to invest in standardized tools to make my work easier).
I did setup my tools for the 10x10 cm version for the time being…
I made myself a jig for the paste stencils that fits 2x 10x10 cm pcbs (front & back at the same time) that I can adjust a little sideways to make fit (I order one stencil with back/front on it).
Then I have a couple of population stands that also fit the 10x10 boards and can go into the oven with them.
The board layout (or several of them) then only has to fit into this footprint (complete with alignment holes for 2mm pins that will go through to the stencil):
z_PCB_100x100.kicad_mod (2.7 KB)
My next project involves a board that is bigger than that, so I have to retool a little.
Will then be 10x16 cm.
If interested I can take some pics of the jig and holders later tomorrow.