Hello doublest -
Our site is not able to provide shipping options that are not available on eBay, as the information we provide to eBay must match their existing options.
However, if you take a look at the USPS site shipping calculator, the Large Package option incorporates the cylindrical shape and suggests that the dimensions used would be Height, Width and Length, where width is the diameter of the cylinder. So, if a Flat Rate option would not work for you, you can use that to define the dimensions of your package and should be fine.
You can see this on the USPS site at: http://postcalc.usps.com/default.aspx when you click on the pop up for package.