Tissot's Indicatrix is based on a set of equally sized circles on the surface of the globe. Different projections distort them in different ways, either changing their size or their shape, or both. The ones here are set out at 30 degree intervals; their diameters are 10 degrees of latitude (but more than 10 degrees of longitude except on the equator: at the poles they cover all longitudes).
Circles self drawn 2006 - Henry Bottomley.