Hi Chrisrolinski,
I am not sure I understand the issue. I think you can make your picture say A5 on Powerpoint and this can be enlarged to A0 by the printers. In fact I think you can make the poster also in another programme and as long as the proportions are right it can be enlarged by the printers to poster level.
I think it may work better the other way round: write on a flipover chart the text and draw / put pictures on it, if you are happy with this then condense it into a smaller proportion.
I think in general it is better to put less info on a poster than trying to fill it up completely