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”.