I've made myself some cool "compasses" for re-scaling any sort of reference material. One of them have scale ratio 1:2 (probably should've done 1:3 or 1:4 too), other one is 1:10 (perfect for anatomy sheets printed in A4 format). It can even be used to rescale minis, for example 28:32 to go from 28mm to 32mm (and vice versa).
They also can be used as regular compasses (for 1:1 measuring)
It work on very simple principle (http://en.wikipedia.org/wiki/Similarity_%28geometry%29#Similar_triangles
I can use 1:10 to scale between millimeters and centimeters (for those unfamiliar with metric system 10mm=1cm and since A4 sheet is about 29 cm long I can print 28cm body (missing top of head) and scale it to 28mm)
Here are some basic guidelines how to cut parts (i strongly suggest making harder one for drastic scaling, in my case 1:10 [I would start considering it from 1:5], while easy one is ok for less drastic ones [anything below 1:5 really]) and how to measure them.
X is Your desired scale, Y is scale You want to resize. A and B are in proportion, so they can have any length (I picked sizes of 50-100mm and 15-150 mm because they were easy to measure and IMHO 15-16,5 cm tools are in exactly right size to use them comfortably.)
Some of sizes I suggest (all of them are between 15cm and 16,5cm):1:2
I've made mine from piece of clear plastic (a bit rubbery, it was cover of old notebook) with joint from copper wire.
I hope You would find this tool useful.