For snapshot size, they generally should be about 3 times the size of the changes (differences). As an example, if you had 50GB in changes during the selected time period, your snapshot for that time period should be about 150GB. From another perspective, they should be about 10 to 15 percent of the total space in the Logical Volume. These are not hard and fast rules, but a good rule of thumb is to leave plenty of extra space in your snapshot for growth. You can monitor how much space you need for data differences and adjust the size of your snapshots accordingly.