The footprint of the concrete pad underneath a Fiberglass Manhole is recommended to extend a minimum of 6-inches [15.24 cm] on each side, past any portion of the manhole or any structure integrate into the manhole.   

Additionally, the concrete pad should be of sufficient thickness to ensure that the manhole does not float.  Anti-flotation sizing should be done by a professional engineer with adequate knowlege of the soil conditions, water table, etc.