> i'm using the doc class slides, and am unable to change the size of the left
> margin (and the font size as well, but thats not an issue just now).

The problem is landscape. Since you are turning your text, changing \oddsidemargin means your text goes down. Try changing \leftskip inside the landscape environment. (But I'm not sure if it is the best way, perhaps the documentation of lscape can help you).

And I have no problem in changing the font size

\documentclass[titlepage]{slides}
\usepackage{graphicx}
\usepackage{lscape}
\linespread{1.2}
\begin{document}
\begin{landscape}
\advance\leftskip-1cm\relax
text text

\LARGE text text
\end{landscape}
\end{document}

works fine for me.

Ulrike Fischer