Bigger is better, of course, but I don't think you'll find converging mirrors much larger. And certainly not for a buck each
Rather than a small mirror at the focal point, you'd want a fairly large plane mirror closer in.
You can use it to move the focal point around where you like.
Alternatively, you could put the small mirrors in the shade and set them to a fixed focus, then use the large plane mirror to reflect sunlight onto them at whatever angle you need. You can move the one big mirror to follow the sun instead of all the little ones...