I have no experience of estate agents overall house size measurement, but their room measurements can definitely be way off.
When I sold my own place, estate agent threw up an ad with a copy and paste job from another sale. It bore very little resemblance to my property.
I think properties are selling themselves recently so some estate agents are lazy around details. I regularly see ads on Daft with obvious incorrect details eg: an open fireplace where there is none, or incorrect aspect given for the garden. I imagine size measurements can be incorrect too.
Never trust an estate agent ads 100%