Whats the biggest size engine on a motorcycle?

The Dodge Tomahawk has a 8.3 liter V-10 out of a Viper. That's the largest I know of.

Boss Hoss is up to 6.2 L V8 (Corvette engine), I don't think anyone will top that except maybe one offs and customs. Japanese bikes are all under 2 liter I think, Harley's too. Go here for more info - they are real, I've seen them on the street. http://www.bosshoss.com/products.asp

The largest engine, designed for a motorcycle (rather than a car engine grafted into a frame), is on the Triumph Rocket – the 2294cc triple.

http://www.bosshoss.com/view_bike.asp?x=BHC3LS3SS

I am with Tim D The Triumph Rockett at 2.3l would have to be about it for a production bike. . As for the others. I cant say it fair to include them as the sky is the limit. Take an engine and somehow attach two wheels and a seat. Trouble is its not a realistic adventure. So my money is on the Rockett. I sure would love to take one for a ride. Apparently in any gear, open the throttle and your gone. It don't wast time complaining,

Tim D is correct if you're talking about production motorcyle engines. But once you enter the world of custom bikes, anything is possible. Renegade Motorcycles offer a 2410cc V-Twin (http://www.renegadeaustralia.com/Engine.cfm) that just beats the Triumph RIII.

These aren't street bikes, but awesome to look at. Up to 48 Cylinders ;-) http://www.transportspecs.com/7-8-9-10-12-24-and-48-cylinders-monster-motorcycles/

The common 'big' bikes are around 1000-1400cc. There are obviously bigger, although most without making much sense for a practical bike.

Around 6200cc I believe. xx