It states in our contract that the plot size ( 260 sq. Mtrs.) is approx so it may be difficult to argue the point with Masa. I suggest you take the matter up with your solicitor to get a legal opinion.
It is possible that the Plot size includes the footpath between the wall and the Road -- impossible in the UK , but then ,this is Spain after all. When we were last there in May06 , there weren't any walls built, so we couldn't take any outside measurements.
Hope this is of some help.
Roger and Fran (Res) Open 15.54