There is no standard size to define what is considered a 'baby' sized salad green - at least I haven't seen one yet. So I made up my own set of parameters based on observation, salad eating and a few reliable sources. Johnny's Select Seeds suggests harvesting salad greens at 4-5 inches tall which translates to 10.16- 12.7cm. I feel like Eliot Coleman of Four Season's Farm wrote that he harvests salad greens at 4 inches, but since I can't find that anywhere in my notes I could be wrong.
For harvesting and data analysis I consider lettuce and brassicas in the 8-12cm range a good size for baby greens.
Currently, I consider 6-10cm perfect size for spinach. Unlike lettuce and brassicas, spinach has an official size: the size of a soup-spoon (again I can't find my notes on this right now). I don't have real soup spoons, but hopefully a tablespoon is an ok substitute.