Funny, you should be asking for 'normal' tire size. 35/12.50R-20 IS the normal size. "P"sizes (metric) are the new sizes. So to speak. At least, for us old farts. Anyway, if I can help you compare...
Old School...
35= 35" tall tire
12.50= 12.5" tread width
r=radial
20=rim diameter at bead.
Metric conversion for comparison...
P= metric "Passenger vehicle"
265=265mm tread width
70=70% profile - this means the bead to the tread face is 70% of the tread width, or 185.5mm
17=17" rim (nice they left something so we could understand.)
So, to calculate the height of the tire...
[(185.5*2) / 25.4] + 17= 31.61
Stock P265/70R-17 are 31.61 inches tall and 10.43 inches wide.
Also, to convert your 35X12.50-20 to metric
35-20=15
15/2=7.50" profile or 190.5mm
12.5=317.5mm
190.5/317.5=.6 so your profile is 60% of the tread width
P317/60R-20 (317, 318, whatever it takes.)
As far as wheel size, the tire manufacturers have their own suggestions for rim width. And, of course, you also have to consider backspacing.
HTH,
EJ