I'd say that is mostly a stylistic choice, really. As far as I can tell, the lenght of the space was only really standardised in the sense that typewriters only accomodated the one space bar - and if that's an en-space, it's just because if you put two, you should have an em-space, so it makes sense to choose the en.
Not sure what grammar should have to do with it, though. If anything, it's something you could find in a style guide, but those generally have little to do with grammar, but everything with maintaining a uniform style.