This is already supported. Drag-and-drop the edge of the circle to resize.

The default size is based on available accuracy, so if it is large you probably need to perform Wi-Fi training.

This and more is documented in the context aware Help under the Menu

There is a third party plug-in that accomplishes this. The “Locale Plug-in Appli Detection” is available on the Android Market in all countries that support paid apps.