Snapshot - how much space will be used?

I have a Windows SBS 2003 VM (on an ESXi 5.0.0.0, 504890) which will be migrated to SBS 2011. Before I start the migration process, I would like to snapshot the SBS 2003 VM. Just in case the migration fails, I can then go back to my previous snapshoted version. Before the migraiton, all data changes (mail, file, print) will be stopped - basically the network will be put on hold. The current VM is around 500GB, how much space do I need to calculate if I snapshot the VM?

Thanks for your feedback.

Regards

Oliver