The following website gives the coefficient of thermal expansion for MDF as 12µm/m-K (http://www.makeitfrom.com/material-p...Fiberboard-MDF)
That means a 10metre long board will expand 0.12mm for every degree Celsius (or Kelvin) change in temperature. If the temperature of the board increase 40 degrees, the total change in length is only 5mm. That is 2.5 mm at each end. So a 10mm gap all round is excessive unless you are likely to get an awful lot of swelling due to moisture.