How big should the spot be? I tried concentrating on an area of ca. 1cm2. But sometimes I loose awareness of the breath over this spot for a longer time and get distracted. Should i change to another spot when this happens? Should i make the spot bigger?
First thing I'd suggest would be to drop the obsession with the size of the anapana spot, you're worrying about nothing and wasting your time. If you're not too hung up on focusing on anapanasati then why not try focusing on the rise and fall of the abdomen? It's a bigger area to pay attention to, you don't need to worry about specific areas as you're just following the
natural in and out of the breath. If you want to stick with the ananapasati then you're only really paying attention the where the sensations of the air entering, and exiting your nostrils occur. It's that simple.
If you notice you've become distracted from the breath, return your attention to it gently and don't get pissed at yourself 'cause it's only natural. The attention isn't used to being directed in this way and it needs to be trained like a puppy, that means being firm with it but not aggressive or overly forceful. The same thing happens to me sometimes and I've got pretty strong concentration abilities so don't expect to perfect it in a week, stick with it and you'll see the benefits over time.
If i concentrate on the nostril are or the tip of the nose something strange happens, which i can not intepret.
I get a pressure sensation which starts at my nose area (bridge) and continues to grow until it reaches the frontal sinus (between the eyes).
Third eye activity, most likely. Is there a sort of "fluttering" sensation happening in the area?
When i concentrate on that pressure it continues to become stronger (sometimes it gets really painful) until it suddenly dissolves (or explodes) just to be replaced by a strong tingling sensation which flows over my body for a few seconds or so. After this event the sensations over the anapana spot get much clearer for some time (maybe 10min) until i get distracted again.
Sounds like you may be hitting 1st jhana already, particularly the fact that you focus on the tingling (a sensation I associate get with 1st jhana). I don't know about the painful sensation, although the pleasurable sensations in 1st can often become rather annoying after a while but I don't know if that's related in any way.
What you're saying sounds like you've probably got some fairly strong skills already, but it sounds like you're getting a bit closer to vipassana if you've noticing that level of detail in sensation. In samatha you're just staying with the object, not looking at the details.
Hopefully that's of some help!
Welcome to the DhO.