How do you calculate FWHM of the primary beam of the telescope from wavelength and dish size?

Resolution angle
Theta = 1.22 Lamda over D
Lamda = wavelength
D = diameter
Use the same units for Lamda and for D
For very small angles...Theta = sin Theta...see trig books
Read down for variants of the formula
http://www.universetoday.com/guide-to-space/telescopes/telescope-resolution/ . . . . .
The usable field of view varies with the quality and size of the collecting surface and for very large telescopes is only a few minutes of arc.
For visual telescopes the Dawes Limit is 4.56"/D in inches = 114"/D mm
Where " = seconds of arc.
A mirror or lens 4.56 inches in diameter resolves 1"
That is for a specific set of criteria...two close 2nd magnitude  stars in a dark sky background with good seeing.

It's not "field of view" it's "full-width at half maximum" beam size.

The field of view is set by aberrations and field curvature.