In a nutshell:
From the dashboard, choose the Configuration/Settings icon. You must be a "master" user under the account to modify this information.
Click on the Lot Type Manager option.
This may be a bit confusing, as it's really a Grave Size manager. But it's been named this from the beginning, so we left it named this now. This is where you define the size of the niche, grave, etc. of the area where the interred will reside.
When you initially get to this page, you are presented with a default grave size. You can either Edit a selection in this list, or create a new selection by choosing Add New Lot Type.
When the data entry page is displayed you can see that you can provide a Lot Type Description (and again, this is really for the interred single location, not an entire lot). You might input "Niche" or "Average Grave", etc. Then you can determine the sort order (determines how this information will appear in dropdowns and reports), the size (could be tiny for a niche, or large for a vault/casket, and can be empty if you don't want to use it), and lastly the price that a purchaser would pay for this space.
Click on Save when you're done. You can also Delete a lot type if you like, but only if no records are currently tied to it.