the size is hardcoded and not configurable via config-file/gui.
I'm not too sure wheter we want to increase it in general (2k is quite a lot to append to each mail that leaves your system, especially if it's html-styling-data).
You can open an enhancement-request at https://bugzilla.proxmox.com - for further discussion (also if others need that too and say so there we could reconsider)