A very big one ;-) No serious I would aim for at least 1,5 meter in your area. Perhaps with lots of luck 1,25 could work on some vertical transponders for you.
I receive the verticals quite ok with a little bit of weather reserve. Some horizontal ones on the edge. A bit of rain and they are gone. 1,25 would be perfect for me and I would receive all the horizontal ones but I'm much more south than you.
I cannot tell you for sure if a 1.80m Offset or 2.40m Prime Focus dish would suffice for the permanent reception of all the horizontal transponders but have a look on the report here: http://forum.digitalfernsehen.de/forum/showthread.php?p=586318 As you can see the signal is not strong even with this 1.80m Prime Focus located between Kassel and Göttingen and since you are 500km more to the northeast from there this is the minimum dish diameter i would recommend. Could be that you are lucky and still the same conditions are there but you never know until somebody close to you has tried it before. For the verticals a 1.20m could suffice as in Hamburg but also here i wouldn't start below 1.50m or try Arabsat 3A of which i know that people can get it even in Stockholm with nit any bigger than 1.50m.