A Pinwheel pattern is simply taking two tiles of different sizes and using them in a repeating pattern. The smaller of the two is "surrounded" by the larger. In order for this pattern to have a pleasurable appearance, the smaller tile should be at least half the size of the larger tile.
For example, a 6x6 tile and a 12x12 tile are used in the following pictures.
For a more dymanic look, a 3x3 tile could be used with an 18x18 tile.
The options are endless...
In these photos a 6x6 "deco" tile was used with an 18x18 field tile.