preview symbol as bitmap (or image)

Hello to all

I have found some discussion on previous forums on how to display a ISymbol as a bitmap. The problem is that it requires the size of a bitmap to be given to the conversion method. I am trying to export it as base64 string, so I use for reference the one that gets exported when you publish a service. Does anybody know how this size is determined and where (in which interface) is it stored

Thank you for the replay
Andrej Trunkl