Is there any way to set/change the font/font size for text displayed using the MessageDialog routine?
No. The MessageDialog routine accesses the MessageBox API, and this API has no way to change the font or font size.
You could code your own dialog and use it instead. I had code where I did that, it might be publicly available somewhere, I'll take a look.
In addition, Windows 7 has something called a Task Dialog, that is similar to, while much more flexible than, a basic message box.
But there is no access to that in ooDialog. Basically on my TO DO list, but that list is very long.