Did you check that you are really transferring the ePub file to the device? The inability to resize text sort of implies fixed layout, thus PDF... you can select your preferred ebook format in the preferences and the priority and which formats to send in your devices' interface plugin (also found in preferences).
The font size of the ePub file can be influenced using the layout tab in the conversion settings. However, I can't help you with that, as I've never had to mess with it. The manual
might be of some help.
As for selecting the device, I'd try out one that has a similar screen size and resolution to the one you have.