I've been summoned? Thanks for the name drop, @
RobTC, glad to know my name is being remembered.
@
HaloSam15, what you're inquiring about, I presume, is the scaling and printing of your pepakura files. There's a few tutorials I'll refer you to for detailed explanations, but I'll also give you a short walkthrough for completeness' sake.
Measuring point-to-point distances in pepakura:
http://www.405th.com/f21/info-how-measure-size-your-pepakura-project-42098/
FAQ for pepakura (in general):
http://www.405th.com/f21/faq-papercraft-pepakura-40607/
Another pepakura FAQ:
http://www.405th.com/f14/ultimate-pepakura-faq-3008/
Scaling tutorial for Halo 3 MJOLNIR Mark VI:
http://www.405th.com/f19/halogoddess-how-scale-more-tutorial-31627/
Scaling tutorial for other armours:
http://www.405th.com/f14/how-scale-your-armour-tutorial-v-2-a-25588/
When you open your pepakura file, if the 3D model has been unfolded by hand in a decent manner, you're off to a good start. You'll know it's a bad unfold if you've got a handful of ridiculously big and complicated pieces and an army of tiny, impossibly small ones. A good one is going to have almost entirely moderately-sized pieces that don't stretch over multiple pages. It would be rare to find a bad unfold, so come back to me if this is the case.
Next you're going to want to scale your armour to your body size using one of the aforementioned tutorials. Once you've done that, you'll want to check under File > Paper and Printer Settings, and make sure you're set to use the right type of paper (either Letter or A4 depending on your country/region, unless you happen to have some really strange printer that uses really abnormal paper sizes).
There's a good chance that doing these two things has forced some of your pieces out of their original positions, so you might have to spend some time rearranging things. If you right click, you can switch to 'rotate' mode and reorient pieces to fit more on a single page (in printing my ODST shin guard, I had to do this twice - first when I scaled the shin up to my proportions, and again when I realized I had to change the paper size).
If you have any pieces that are too big to fit on a single piece of paper, you'll have to right click and select "Join/Disjoin Face" and click to seperate along faces. Do this repeatedly to break a single part into multiple - and remember, you can do the opposite, too, should you feel like it. Unless you've had to upscale the part AND switch to a smaller paper size (I think letter is smaller than A4), you shouldn't have to do much, if any, of this.
Then make sure "show flaps" and "show edge ID" are turned on in the 2D Menu dropdown and you should be good to go. If you need any tutorials on the what comes next - cutting, gluing, resining, reinforcing, bondoing, or painting - I recomend you do some research on the forums. Depending on how in-depth you are, this could take you a few hours, but I guarantee you all the information you absorb will be reflected in the quality of your final build.