@Spencer18 You cannot retro-actively add a security deposit for an existing reservation. When you accept a reservation it is a contract between you and the guest with the terms at the time of the acceptance. On a go forward basis, a good size security deposit is especially needed for an entire house as you are not there to monitor guest behavior.
I don't know if hosts have a rule of thumb in terms of the size of the security deposit. Remember the security deposit is not taken at the time of the reservation and only accessed if there is a substantiated claim. Some hosts feel a large security deposit makes there listing less desireable. I agree, but I think about the travelers who would find it a deterrent and those are the travelers I want to weed out. I would recommend you have at least a $2000 security deposit. Maybe more...