Converting from 16:9 to 4:3 format, keeping font size

Hi,

I've got a presentation in Keynote 6.5 which I'd like to convert from 16:9 widescreen to 4:3 normal format.

When I change the document size accordingly, a lot of my text boxes and formatting 'jumps' all over the place... a 28pt font becomes 16pt, text boxes become smaller, etc. Bacially, the entire layout gets messed up.

I tried entering a manual 4:3 ratio, going from 1920x1080 to 1920x1440. Same thing. 28pt font now becomes larger. Layout again gets messed up.

All I'd like to to is keep the width, but add some pixels to the top and bottom. And keep text sizes the same. Is there a trick of how I could do that?

Thanks very much for your help!