[OS X TeX] proper font scale for A2 size

[Konstantinos Vasilakos]

Hello,
I am using the baposter class for making a poster and the paper size is A0 rather than what I would like to be, an A2, 
there is an option in the class that I can scale the font in order to appear correctly when you change size papers.
I can do fontscale=0.5, but how can I be sure that I am using the maximum font scale value the A2 can handle properly, or is there anyway to change the paper sizes  it without changing the font scale ?
Thanks
K.