When reading a book bring up the menu bar at the top by pressing near the top of the screen, then tap the little button that looks like "Aa". This will give you the option to set a larger font size, as well change colours, set margins, etc.

I'll try to add an option to switch to landscape mode in a future version, this might take a while though.

In terms of magazines there's nothing I can do really, that's all down to Amazon's side of things I'm afraid.