LyX ⇒ Change font size of the math

[Davinci]

I am trying to put a 20x20 matrix in to a lyx document. But obviously this matrix is too big to fit. That's why I would like to change the font size.
I tried \DeclareMathSizes{12}{10}{7}{5} in the preamble but it doesn't seem to work..
Is there another way to change the font in the math mode?

[frabjous]

The relsize package has \mathlarger{..} and \mathsmaller{..} commands, but I've never tried then with LyX, nor with a matrix. You could also consider using \scalebox from graphicx.