1. First, just because the pdf looks small on your computer does not mean that it prints out that size. Try opening the PDF, doing PRINT, set the print options to 100% (it probably opened with FIT selected) and it should tell you how big the document actually is. Usually you can just have a large-format print-capable shop print the file and it will come out correct as long as they can do a true 1-to-1 print.
In addition, some plans will have a scale marking on them (i.e., |_______| = 1 inch) or indicate the actual wingspan of the model. Either can be used to check the print out to ensure it's accurate.
2. If it's a true scratch build, one would normally research the aircraft to find out what size tires it used. If its a plan build, then the plans would normally have the wheels shown on the plan and you can measure them. Generally, if the build is at say 1/4 scale, you would find wheels that most closely matched 1/4 the size of the full scale or the size of the plan's wheels. Pilots are a bit of a crap shoot. Again, just find one closest to the scaled-down size of your model. Generally, WWI pilots are slightly shorter than WWII pilots and are shorter than modern pilots.