Ah, if swap size = RAM size is sufficient for suspend to disk and hibernate, then the default proposal is correct: 2 GB swap for a VM with 2 GB RAM. Is this the reason, why the "Enlarge Swap for Suspend" checkbox is deactivated? The hibernate problem - well, after some investigation it looks like VirtualBox does not support sleep or hibernate for guests. Well, then I will use VirtualBox's save machine state functionality instead.