I also encountered the same problem.
My fix was to install PRS+ so that I can gain control over the epub format. PRS+ allows you to set css to customize the epub format.
A great write up by a forum member on installing PRS+ and customizing css
http://www.anamardoll.com/2011/06/er...tion-with.html
I added my own code to control font size:
body {
font-family: "Charis", serif;
font-size:116%;
line-height: 140%
}
"font-size: 116%" means to enlarge the font size by 16%. Line-height affects the spacing between lines. You can play around with both.
Using this, 90% of the time I use the Small size, since that font size is now enlarged. (I agree that it was too small as a default and was useless). Occasionally I go to M size. I never have to use L or bigger anymore. The font size in Small is about 10-points.
I find that the font sizes are pretty consistent with the PRS+ installation and font change.