Will it fit on the projector screen?This is a very important question. Before giving a talk, try to figure out the projector resolution.
(I think most of them are 1024x768.) I once got the projector replaced just before my talk,
so I found myself trying to edit the .css files right in front of the audience.
xte 'mousemove 1024 768'
The easiest way to adjust the width of the slide is by using the width parameter of .maincolumn.
Alternatively, you can change the font sizes. If you do decide to change the font sizes, you might also need to adjust the formula size in the headers of the .scrbl file, and perhaps in the @fsize function.