T Olive.
There is a big problem concerning ownership and land size north of Tejo River in Portugal. The
local city halls cannot do all the necessary land registry so not all lands are registered by the owners and a lot of owners think their land is bigger than it actually is.
Adding to this big problem is the money oriented+ lack of ethics of a lot of real estates that don't care about the posterior problems regarding the buying.
I've bought a farm around Castelo Branco and saw 3 or 4 farms with a lot less area than advertised but normally the people showing the farms give an aerial photo from google maps. Maybe not before but during the showing i'm sure they give the reason being what Burra Maluca said.
Since you are a foreigner and live abroad, when you contact them, ask them for the documentation and a google aerial photo so that you can check before hand the size of the farm.
Next time you come, give me a shout and i'll try to help as i can. Take care