This is completely off the top of my head so...

The sarsens started as a 'blanket' of stone.

With Africa crashing into Europe and the formation of the Downs - The chalk being pushed up to the surface by the ripples that crossed Europe from that crash (Starting with the lifting of the Alps) The sarsen was lifted to the surface with the Chalk.

This process and further natural process's broke the Sarsen down into smaller pieces. Local prevailing conditions would then condition how small they were broken..

Point being..

Isn't it likly that the dimensions of the stone would be similar in any given area, seeing as the forces applied to them would be similar?.

Tony