This is a very good topic indeed .
Many parameters surrounding all to stand up paddling 's purposes on a surfboard had been described ; Excepted one which is the ratio of volume / surface of the board and how their distribution is organized. That parameter of ratio area/ buoyancy is the key to do the most functional and fun design .
To explain shortly : there is more stability to stand up paddling on a piece of plywood rather than standing on a rounded buoyant .As well you might standup longer on a piece of plywood before sinking than the buoyant still floating at the water level. In last that's more fun to surf riding a plank shaped as a skimboard.
Nowadays there is more too big SUP rather than too small SUP. The prime question to be asked would be : When do your SUP is too big and what is the real satisfaction in surfing even standing up paddling on a surfboard.
To my opinion there is no satisfaction if no challenge does exist
