Sentence:  The surface of the Balaton and the surrounding marshes is not less than 24 German square miles, or 384 English square miles; its principal feeder is the Szala, but all the water it receives appears inconsiderable relatively to its superficial <WSD>extent</WSD>, and the quantity lost in evaporation. 
Meaning of the word marked with <WSD> tags:  The space, area, volume, etc., to which something extends.