The following options are available for each avatar style.

radiusrnumber0Avatar border radius
dataUriboolfalseReturn avatar as data uri instead of XML
Not supported by the HTTP API
widthwnumbernullFixed width
heighthnumbernullFixed height
marginmnumber0Avatar margin in percent
HTTP-API limitation Max value 25
backgroundbstringnullAny valid color identifier
HTTP-API limitation Only hex (3-digit, 6-digit and 8-digit) values are allowed. Use url encoded hash: %23.

The avatar styles may offer additional options. You can find them in the avatar style documentation.