How to calculate the sail size

[Cmarois]

Hi everybody,

I have a sail that I wish to sell, and I'd like to provide the size of the sail ( which I don't know and it's not indicated).

I originally searched online for the methods, which I did not find, so I guessed it was similar to calculate the area of a triangle.

So here is what I came up with:
My sail is 13.5 feet tall, 7 feet at the boom. I converted that to meters which gave me 4.11 meter tall and 2.13 meters at the boom.

After doing the calculation : (4.11*2.13)/2, it comes down to 4.3 square meters.

so I thought my sail was of size 4.3. But someone came and compared what is supposedly a 4.5 sail, and it's half the size of mine.

So therefore, how do I calculate the size of the sail, since apparently it's not what I thought.

Many thanks in advance.
Chris.