The mention of half an acre could be to emphasize the speedy nature of Jonathan’s fight.1 Alternatively, it is possible to translate the phrase so that it describes the manner of the slaughter rather than the area. For example, the New English Bible, in that first attack Jonathan and his armour-bearer killed about twenty of them, like men cutting a furrow across a half-acre field.
14 And that first strike, which Jonathan and his armor-bearer made, killed about twenty men within as it were half a furrow’s length in an acre of land.