hello everyone! Few games before, i had an strange situation. My AHD ninja was prone on the high box close to rhe corner (but not touching it). On solid distance there was an Tanko which head (silhouette =)) was below box roof. Then the enemy said that S0 is have a height of standart base (+-1 mm), so he can easily see me. so, have s0 even the smallest height or not?) sry for my bad english)
An S0 is the height of a default base. I forget the exact dimensions but it's possible to eyeball; there are many situations where even being back from the edge of a building's roof, you can still be seen by someone on the ground.
The wiki is a massive PITA to go through, so i'll just recap what I know from somewhere in a rulebook. Prone and at higher elevation that your opponent (i.e: opponent has to 'look up' to see you) grants you partial cover. S0 is defined as 25mm x 25mm x 3mm (so as others have said, the dimensions of a standard 25mm circular base). As mentioned by @spears and @ijw an S0 keeps the width of it's default silhouette, but changes height to 3mm.
Where is that definition from? Afaik you retain your original base width, buggered if I can find it in the wiki though.
S0 doesn't have a fixed diameter, although I strongly suggest using 25mm for items that are 'only' S0 like Mines. From http://infinitythewiki.com/en/Prone Prone troopers have a Silhouette (S) value of 0, that is, the equivalent height of a base. Cover on high surfaces is there as well: Prone troopers that are the target of a BS Attack declared by an enemy from a lower position than them will benefit from Partial Cover MODs against that attack.
So S3 or bigger going prone would have what kind of size? Do they actually get smaller? or is only the height adjusted? Never really thought about it..
The physycal dimensions of the bases never change, only the Silhouette value of the trooper. Going to S0 means "becoming" the base, whichever that is. Now, there is a part in the rulebook in which this change of S allows access through doors (Narrow Access, Cargo Access, etc...) you couldn't when not prone, but that is not related to the inches/cm of the model's base.