To the extent of my knowledge, there's no real "proper size" for system restore reservations. It can be pretty small if you're not using shadow copies (Previous Versions) as then Windows is only making restore points with configuration changes, the bulk of which is registry and settings with some files tossed in. Windows will manage the system restore space pretty well on its own, but I find that on a 60GB SSD with 40GB filled, I only "need" a few GB to encompass everything including shadow copies. One normally only needs one or two prior restore points. So, just let Windows do its thing is my take and if you need more space, decrease the reservation.
If you feel that isn't enough, I'd suggest augmenting with Windows Backups and system images on a second or external drive. And/or a 3rd party backup program like Crash Plan, Carbonite, Mozy, Drive XML, etc.
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