In order to improve operation on active fire detection, geostationary satellites as GOES, MSG and MTSAT are already operative and they have led the international community to think that the global observation network in real time may become a reality. The implementation of this network is the aim of the Global Observations of Forest Cover and Land Cover Dynamics (GOFC/GOLD) FIRE Mapping and Monitoring program, focused internationally on taking decisions concerning the research of the Global Change. The estimation of the minimal burning area detected by MSG-SEVIRI is analyzed in this work; it's a very relevant issue since it implies the sensor's operational availability in active fire detection. This minimum size is conditional on the fire location inside the pixel, as shown before from the theoretical point of view, among other factors such as atmospheric conditions, fire temperature, among others. A spatial analysis has been carried out, analyzing several parameters referred to spatial performances of MSG-SEVIRI sensor as pixel footprint in the 3.9 microns spectral band. In order to obtain results based on real data, a large data base focused on the active fire detection in the Iberian Peninsula has been studied. Statistical results obtained from a large data base of fires detections, show that the probability to detect a minimal size of 10ha is 90% and 4ha with 50%.