You mean for text bubbles or separate panels that just contain text as to move the plot forward?

For text bubbles, try and keep to 6 point fonts or bigger for readability, that would count for any kind of comic font, a more rigid font, or of course hand-lettering.

For panels only containing text... It's a bit of an older way to tell stories, and some more classic Franco-Belgian comics still use them, but for the most part you want to show as much and talk as little as possible.

Hope this helps you a bit, but please ask more if you want to know more! ^^