If you want your sum's subscripts with the font size of a subscript, do the following:
1. select layout-->document-->use AMS math package;
2. write:
\[
\sum_{\substack{0<a<n\\
0<b<m}
}a*b\]3. highlight what you have just written;
4. convert it to mathed mode by then selecting Insert->Math->Inline Formula or the appropriate shortcut (M-c m) if using emacs-style bindings.
I hope this helps you!
Paul
