I have always dreamnt of living in a large mansion close to a sea side
That sounds amazing. The word itself is interesting. Comes from Old Fench from "mansio" - "dwelling"
Any other facts you know?
For example the English word "manese" meant a property big enough for parish priest to maintain himself. It's no longer sustained this way but it used to be.
What what determines a mansion?
Pretty much wealthy dwellings. Luxurious, extensive properties. It started in ancient Rome and then followed through the ages.
When was the term first used historically?
That's not known exactly, however the oldest known mansions are known form Rome. After its fall building unfortified villas ceased.