I just wanted to know how i can convert the file into a A3 size
At the moment we have not yet implemented A3 size, I'm afraid, but it's in our backlog. I will send you a note if this size appears.
As a workaround would you please try A4 size? That will give you the same dimensions, just choose a resolution big enough so the printed version is looking good.
To choose the A4 size click the size option in the frame bar like so:
I hope this helped.