Ah, I found some of the info from the post I spoke of regarding tombstones at Aarin's Spectacles, a Sims 3 blog (I won't link it, as I'm not sure if it's allowed...plus there are plenty of mods/cc etc there), and this is from her FAQ:
"What determines my sim's tombstone size at death?
- Tombstone size is purely dependent on lifetime happiness points. The game counts both spent and unspent points towards your total. You cannot edit the tombstone after your sim has died. They get what they get when they die.
NUMBERS AND TYPES OF TOMBSTONES:
LESS THAN 75,000: small, flat, on the ground
75,000-150,000: small, standing
150,000+: large, standing"
Hope that helps!