personally I scale every armor part separately by measuring each limb length and torso width from armpit to armpit.
I hope that helps.
Others will no doubt pitch in for measuring an entire 3D model in pepakura designer (laellee among others is good at this stuff); there's a trick to it that I cannot remember, although it is listed under "change scale" under the tab "2D menu"...
Thanks for replying , I'm kinda beginner level so could do with the help lol. I'm lead to believe most Peps are scaled for somebody around 5'8 so just need to work out the percentage I would need to increase by to fit my 6'4 body
Your best bet is (as a beginner) to scale each piece, using Pepakura's point-to-point measurements to guide you. For instance, for a bicep, check the width at the widest point and overall length, and then scale according to your measurements. In a perfect world, you can just go by what length looks good for you on your arm, and then hopefully that scale also has enough girth at the middle to fit your arm. If not, then scale up. I'm 6'3", so I appreciate the scaling aspect when it comes to 5'10" scaled armors fitting 6'+ frames.
Now if you are lucky on the particular pepakura armor you are looking at, you can import all of the pieces into the software and they will still be oriented in the correct positions (rather than all piled on top of each other). If you do get lucky, importing all of those parts will give you a head-to-toe measurement to start overall scaling from; if you can pull an overall height that way, you may get lucky and get 90%+ of your scaling usable just by doing that. Scaling a 110% on a standard 5'10 armor would hopefully get you pretty close for fitting at 6'4. Of course keep in mind though, it comes down to the armor and how well it fits in the first place, so it pays to still double-check your parts. As an example, correct proportion Iron Man armors are notoriously small in the waist/cod area to the point where a screen-accurate armor pep just scaled overall to a 6'2" guy would still only have a 33" waist or so; great for wiry guys, not so much for most other people.
Last suggestion I can make, if you are still having problems scaling, you can commission someone to scale the armor for you. I do offer this service, I work up a digital mannequin off of a set of 20 or so of the wearer's measurements, and then rescale the individual pieces to 'fit' the armor to their mannequin. It can save a lot of time and parts rebuilding; having said that, if you are just getting into Pepakura and buidling, I would recommend trying the scaling yourself first and see how it goes. You'll learn a lot more about how to use the software that way.
Best of luck on your build!