Grain size does matter for the healthy development of roots, which also depends on the type of plants you're considering. Plants with massive root structure would need bigger grains to hold and develop but over the years I've tried using micro grain size aka sand with monster roots hidden beneath and the plant is healthy. Thus I've come to the assumption that so long as roots can hold onto something and there's room for development - its grows.
With that, I do have to caution you against using smaller grain. Over time the substrate would become compact and compress. This makes it harder for the root structure to develop. Get something in between bro. Follow the middle path...
