Why do ooids not grow larger than 0.5 mm?
I was unaware of a 0.5 mm maximum rule with ooid size. The Sam Boggs Jr. book on Principals of Sedimentology and Stratagraphy indicates that ooids can grow up to 2 mm. Grains larger than this are pisoids.  The size limitation may depend on how you define an ooid. To me, an ooid carbonate grain with a nucleus and concentric layers of calcite or aragonite. 

The size is likely a function of the availability of calcium and carbonate ions in the water (calcite or aragonite saturation).  I think that microbial action also contributes to large grain size, but if microbes are involved, then you might call it an oncoid instead. 

The similarity of ooid size is a function of similarities in the environment of deposition.

Microfacies of Carbonate Rocks by Erik Flugel is a great book that discusses these features in detail.