For a parabola to work at maximum theoretical gain (based on aperture size and feed efficiency), you have to be within about 1/10th of a wavelength accuracy. That's about +/- 1.25cm. You can build it much sloppier, but the achievable gain and pattern will suffer.
The feed is also critical as you don't want too wide a pattern resulting in "overspray" which reduces xmit gain, or too small a pattern that illuminates only part of the dish, resulting in low receive gain. I short, you need to match the feed type with the f/D ratio of the dish.
The best source of dish and feed info is at: