Actually I think there is a way to achieve this - you need to use Cascading Style Sheets to define your text size, and as far as I can remember, putting an absolute value in as a text size in the CSS renders the view text size option useless. I THINK that works (it's a long time since I did it because in general it is bad design to stop people changing the size of text) so if it doesn't work just say so and we'll see what else there is...
Stuart