About a 20 liter tank.
http://busdepot.com/details.jsp?partnumber=255070814C

Photo's of the tank just after I got it, one photo of the modification I made
to it to accommodate the air conditioning hoses that are in the same space
the tank installs.
http://www.flickr.com/photos/syncronaut/7019697815/in/set-72157629312427896/

The tank may protrude below the propane tank by maybe 2", but is so close the
the tire and it's not likely to create any kind of clearance issue. So far, so good.

bob � see notes at Flickr site which describe tank modification.