We manage just over 5,000 desktops and laptops with about 3,500 connected at any one time using a 2003 VM and separate VM running SQL 2008. We only use it for imaging.
I find it's fine provided I clean out events over 90 days old after a big reimage event. So after we'd migrated all desktops from XP to Win7 the events needed clearing out and now we're reimaging most PCs as we move to a new building it's slowing down again and will need an event clear out.
I've not seen any official updated sizing info.