I am doing a homework on globular clusters and could not find trustworthy data on the average diameter of a globular cluster. Most clusters I included had a diameter of 120-150 ly. But is that the average diameter of such globular clusters?
Globular clusters vary in size quite a bit. They contain from a few hundred thousand to over a million stars. Probably best way to determine size is to measure its extent across the sky by telescope and then calculate that distance in km. One could also model the size by assuming the number of stars, the average size of the stars, and the spacing based on this size. One could build a globular cluster of 300,000 stars or so and determine the approximate size.
You ask if 120-150 ly is the average diameter of such globular clusters. I'm confused by exactly what you mean but you have sampled a small range of sizes considering globular clusters can be well over 300 ly in diameter.
From Wikipedia, and based on comment from @AtmosphericPrisonEscape, you have "Almost all globular clusters have a half-light radius of less than 10 pc, although there are well-established globular clusters with very large radii (i.e. NGC 2419 (Rh = 18 pc) and Palomar 14 (Rh = 25 pc))."