What size tire is recommended for a Porsche 1974, 914 2.0 L

Go with the existing size, if you want quality, my opinion is Goodyear Eagles or Pirelli's . You want so-so, firestones. You want "Cheap enough to just replace" go with an off-brand.

195/60 R15 if you have the 5.5 Fuchs. (if that is yours in the pic that is the wheel) 205s look nice but are too wide for the wheel and will give you a spongy feel in hard corner. 195/60 are just right!

If I recall correctly, the factory wheel and tire decal had 155SR15 on the early 1.7L cars, and 165HR15 on the later 1.8L/2.0L cars. 175/70HR15 was the optional lower-profile tire size. I don't know about the 6-cylinder cars finished at Porsche. I believe that the original optional 175/70R15 tires are being reproduced by (or sold by) Coker Tire. These are pricey reproductions, fyi. Alternate tire sizes that will not introduce a significant amount of speedometer error are either 185/65R15 or 195/65R15; the latter is readily available from Volkswagen Group of America franchise dealerships.