Problem with tutorial - using units

[philem]

Need a bit of help again guys,
I am following a tutorial to model a Junkers F13, this is a copy of a particular command in the PDF

Wings does not specify dimensional units, and uses an arbitrary dimension known as a Wings-unit. For the purposes of modeling here, let's assume from now on that 1 Wings-unit = 1 m. Now from the right-click menu, right click on the Cube option. Find the dimension that lies on the axis of the wing span in the plans, and set it to 17.75.

I don't know what this is or how to set it to 17.75, I have bought the plan in, bought the cube in but cannot find out how to set these units, I have searched the forum, checked in the Wings 3D manual 1.61, Wings 3D handbook, Wings 3D book, Googled it, checked out a few video tutorials I have downloaded but cannot find anywhere that it mentions setting dimensions.

Can someone help please?

As always
any and all help is greatly appreciated.