Here's what I finally settled on:
As a result, my cones hang horizontally, with the florets opening out, as opposed to pointing down, vertically. But I think they look good. Also, using a beaded necklace rather than ribbon meant I could just count out the same number of beads between each cone to end up with an even placement. (No measuring!)
I used an uneven number of cones (so one cone would fall in the centre of the garland) and eyeballed the sizes to make that centre cone the largest, moving out from the centre with cones in decreasing sizes, and placing the littlest cones on either end.
I knocked a couple of new nails into my mantle and strung it up. Done!
What do you think? Rustic and festive, right?