I would guess 1-3mm gravel would be sufficient to keep glosso, hairgrass and microsword down. But really it depends on the way you plant them. Now Java Moss and Riccia can be tied to a rock/driftwood with fishing line.
It was javamoss, only about 1inch tall,it came in one of those little pots,I took it out of the pots shoved it in the gravel,next day it was floatig about the tank
Think the carpeted tanks look so amazing