How large is a stone? Luckily, there is an internationally-agreed set of Standard Stone Units (SSUs)
Presented in descending order of size
Baby (this is the assumed size of Richard's "Great White Stone")
Football (size 5)
A good portion of a breeze-block
Toy Millennium Falcon
A house brick
Hermione Hand (coincidentally, also the size of Richard's hand)
A football that has been truncated about a third of the way up
A Halloween mask that a child might wear
A big rat
Red Nose Day nose (car)
A third of a house-brick (NB. a entire house brick did not become an SSU until chapter 39)
A partially-deflated child's football ( the ball is partially deflated, not the child)
A small ball (bigger than a tennis ball, smaller than a football. Not the size you're imagining)
Small dinosaur egg
Small Easter egg (the type that would be disappointing if it was your main egg)
A shoe belonging to a medium-to-large ventriloquist's dummy
Half a paperback book (but only a short one)
Potato (you know the sort of potato)
Rock cake (which is a cake the size of a rock. A bit recursive, if you ask me)
Half a tennis ball
Half a tub of chewing gum, chewed up
A squash ball with a ping-pong ball squeezed onto the end
Red Nose Day nose (for a human, not a car)
Chicken egg (and Cadbury's Creme equivalent)
A testicle of an old man
Slightly larger than a grain of sand.
*Size can vary, which limits usefulness as a measuring device.
All these sizes of stones MUST be cleared.
The three-parameter scale
In Chapter 24, Richard proposed the three-parameter scale for stone sizing, which is designed to remove some of the confusion of the traditional SSUs.
After all, how are you supposed to know the size of an egg? It's virtually impossible to imagine an egg in your hand unless you actually have an egg there. But with the new system, the exact size of each stone can be accurately communicated. Richard toyed with the idea of a fourth parameter for "age of stone", but abandoned this as it would be impossible to determine without access to specialist geological tools.
Here are the agreed sizes, from largest to smallest:
For clarification (not that it needs it, because it's bloody obvious), the three sizes do not refer to length, height and width of the stone.
If that was the case, a "medium medium small" stone would be the same size as a "small medium medium" stone. Whereas in reality the former is much smaller than the latter.
I mean, come on. This is basic stuff.