Using a bit the dom, I end up dimensioning a byte array stream to write a body in.
Sadly, I don't get any method to retrieve this information.
A quick look at available implementations tells me that SingleBody subclasses rely on String and bytes array. The size information is thus available without effort.
Thus I would find it interesting to expose a singleBody.getSize() method, that would lead to better memory management in James (upon attachment parsing).