... There are is a few ways this can be done, each working slightly different, sofor different situations:
* As a Mapplet
The My Maps content directory includes a few possible tools:
* 'Follow Me' circle
The above link will maintain a circle in the center of a view - at a set radius, change the size by changing the value URL inside the Search Box - opens on an example location
* Fixed circle
Enter a URL like in the Search Box
but putting a valid in the lat/lon parameters, and choosing a radius.
- TIP: this type of map can be saved to My Maps for later use; or you can import the into a My Map, which will add a circle to your features.
Note: the radius param also accepts other units, such as radius=44km&, radius=650meters& and radius=175feet&.
Slightly off topic, as its for Google Earth (in a browser) but there is a nice demo (Polyplot) here: