Round it is then. Conveniently, the 15mm FoW bikes all fit nicely on a 1" fender washer.
Bikes in general are a bit of a conundrum. Are they vehicles, or are they part of the rider (the way 40k treats them)? I'm inclined to treat them similar to how FoW handles them. They're a vehicle that almost vanishes once the troops dismount, and I'll use a single spare bike without rider sitting to indicate the unit's bikes as parked.
Since I am looking at dismounts, I am inclined to treat the bike and riders as distinct models when taking hits, with the caveat that if the bike is destroyed after a move action the rider and any passengers are either also dead or have to make an armor check. A little complicated, but I don't plan on fielding dozens of them, either...