Hi and welcome to iboats
Length will depend on the depth of the river. You want to have at least 10x the depth of the water AVAILABLE for use, though the length of rode you actually use can vary depending on current and wind conditions. 3/8" diameter will work fine for your boat, but don't skimp on quality. Pretty sure iBoats sells pre-made anchor rode- say, 100' or 150' of line, a few feet of chain, and the shackle to attach it to your anchor, If they don't, google it. Plenty of places do.
Finally, 3-strand line is stretchier than braided, so it absorbs shock better. It'd be my recommendation for anchor rode,
My .02