It happens sometimes - if they're enclosing a circular area, and the grass inside is a darker colour, this is what's known as a fairy ring. The toadstools should disappear with a fortnight or so, though they may return next year. Often occurs on the site of previous planting, perhaps an old tree or shrub that used to be there.

What size are they? You will often find tiny brown ones all over the lawn, they are just very small and often overlooked. Known as "little brown jobs" amongst mycologists.....