Of course hog factor would be critical here.
The first (x-1)(5?) submissions might be 1Kb each leaving the rest up to you to fill, and fill them you must!
It could be one long linear work with a size/artist ratio resetting at a certain number of submissions with a hog graph heartbeat along the bottom.
Or maybe it could be a component of the amount of time since the last submission, so rapid posts would have tight size restictions and as it slowed pace, the alowance would increase.
Picture, no smell, artists waiting with sweaty mice poised over the reserve link, watching the byte count achingly tick over. Hoping to outbid eachother for that precious extra few pixels.
Another slight twist in the idea, - do you see this one forming? - the work is limited to 400px high and the time factor determines the y allowance.
Brilliant! simple, easy - JVO ! LET'S GO!
I'm going to bed. I have to launch my employers website at tis time tommorow - 12pm aussi time.