The sizes of G664 Granite headstones can vary depending on the specific design and customization. However, some common sizes for G664 Granite headstones are:

1. Single Grave Headstone: Typically measures around 24 inches wide by 20 inches high, with a thickness of 3 to 6 inches.

2. Companion or Double Grave Headstone: This type of headstone is larger and usually measures around 36 inches wide by 36 inches high, with a thickness of 6 to 8 inches.

3. Family Monument: These are larger headstones that can have multiple sections to accommodate multiple family members. The size can range from 48 inches wide by 36 inches high to much larger dimensions depending on the design and customization.

Its important to note that these are just general size ranges, and actual sizes may vary based on personal preference and cemetery regulations.