Look at the local streambeds and copy them. Unless you find local black lava streambeds, don't try to make it work in your yard unless you are going for a totally modernistic unnatural look. For a natural look, vary the size of the rocks, with some big ones anchoring the bends and interrupting the "stream", with smaller rocks. This is a good one. https://www.pinterest.com/pin/210754457531720328/ You want to avoid the drainage ditch full of gravel look.

I did something similar on my side yard because our houses drain to the street out front. We put down 2 layers of landscape fabric after getting the slope right, then decomposed granite with bull rock on the edges. Most of the granite has washed away over the last few years & we will be putting down more bull rock to replace it. It flows very well in torrential rain, but I do have to spray the area with vinegar quarterly to keep weed seeds from sprouting in there. We did not try to plant that area, but if we had more space we might have put some large potted plants thru there. I'll get a pic later today so you can see it.

Though you were afraid of the smaller gravel covering too much of the larger rocks, that's what needs to happen in order to have it look more natural and better. Too, the overall "path" is too uniform in width and texture. There should be more variety in rock size. To fix this, I would obtain some large rocks/boulders and distribute them randomly ... not with uniform spacing, but with irregular spacing and with some being along the path edge and some being within the path. Reshape the path edge such that it is irregular and somewhat undulating. The large rocks might cluster in some areas and in other area be sparse. Then, reset the medium rocks such that they are also irregular, clustering and in some areas sparse. Have them reinforce the undulating path. Working down in size, sprinkle the gravel among rocks such as to create stream bottom that is solid (no soil shows through.) Obtain even smaller, chip sized gravel to fill in between the other gravel. Do the same with some coarse sand.

Will the bed be used to redirect run-off during rainy times and with snow melt or will it remain dry and just be decorative? If intended to be functional, you want to make sure it is sized appropriately to accommodate the water flow. Typically, dry stream or creek beds are wider than they are deep. And usually by a ratio of 2:1. So a 2-3' wide bed should be 12-18" deep. The rocks you use should be in a range of sizes, just as they would be in a real stream bed. You will need a fair number of large rocks (6-10") as edging along the sides but placed rather randomly. And even a few random larger boulders, especially sited at any curves in the stream bed. For that length and width, you will need 2-3 tons of 1-4" diameter river rock and probably another ton of the larger sized rock (~80-100). The boulders you would purchase by quantity rather than weight.....as many as you need. Make sure you lay landscape fabric along your stream trench (anchor it firmly in place with the boulders and large edging rocks) and then a 1-2" layer of coarse sand. Then fill with your river rocks, evening them out a bit across the surface. You can use whatever shade tolerant plants you like but generally you want lower growing plants closer to the edges of the steam bed and taller plants at a bit of distance. Hostas come to mind immediately, with perhaps some Japanese forest grass (Hakonechloa) here and there along the edge. Primulas are another possibility if enough moisture in the area. Other plants I would consider are astilbe and columbine for some height but you can really use any combintion of shade tolerant plants you like. Pinterest is good for some inspiration on planting layouts. The bottom line is you want it to look natural. So avoid any rows of plants or too precise placements :-)