bestjit.matrix settings for exportimage?

hi ALL, an elementary question for most but not for me.
I wrote a patch to compose a Open Gl scene ad now i'd like o export it as image for a publication. I transfer it to a jit.matrix and then use export figure. The problem is that the quality is low (not so many pixels). I need the highest resolution possible. How do i increase the number of pixels? Which are the best settings (float or char type data influences the export quality)?
Sorry i have an intuitive approach and the jitter matrix is still obscure for me..
Cheers
A/J