Most contracts have a plan attached..if not then you should have had one otherwise you do not know what you are buying
All plans are of course drawn to a scale....it varies but may be 1:100, 1;50 1:75
So measure to inside wall and outside wall off plan and on site....give or take a tollerance there should not be much difference...check your plans
I have had an approach to measure the apartments by one of the buyers but to get involved there has to be a group problem
Now if the marketing brochure states size and the plans are different..even if you have not checked the plans with your contract you may have an complaint
......in UK it is very much 'buyer beware' but in Spain I am told [ and this would need legal checking] that if the brochure says ' a certain size' and it is not built to that size, then you may have an 'complaint against the builder'