9k is the standard for "jumbo frames" on host interfaces, switches must support higher numbers to allow for vlans and other extra header info.
This has a lot to do with the back-plane buffer size, some ISP's support 9K globally (Hurricane Electric).
On a host you should never exceed 9K. You must control all systems end to end or you'll spend more time trying to figure out what packet fits. This is why SANs are typically 9K, it fits a NTFS block nicely AND you control all devices and switches (SAN, HBA and Switch), cutting down on the number of trips to move your data.