Why does the number of unread articles decrease, though I haven’t read anything?

Unread articles lose the “fresh” status by Google Reader after 30 days. So if you have unread items which are older than 30 days, it’s a normal and intended feature of Google Reader to mark those articles as read – which results in a decreasing number of unread articles.

How can I adjust the font size of the preview / inline browser?

You can adjust the font size with CMD + (bigger text) or CMD – (smaller text). See “View” menu.

Can you give me a list of all keyboard shortcuts please?

There’s a list of all keyboard shortcuts under “Help” – “Keyboard Shortcuts”.