Hello o mighty developers!
On the Swedish Wikipedia, we have recently discussed the default size for thumbnail images, which is currently set to 180px. As this is not very much in today's world, and especially the IT-friendly Swedish-speaking regions, people manually enforce a higher resolution (by the size attribute in the image wikicode). We would like to avoid this, and provide a richer experience for our not logged in readers, by raising the default value to 250px. We believe that this is done by adjusting $wgDefaultUserOptions['thumbsize'] to 4 instead of 2, but I suppose you know what you are doing.
The relevant discussion can be found at http://sv.wikipedia.org/wiki/Wikipedia:Bybrunnen#Bildstorlek, and an earlier discussion on the same subject can be found at http://sv.wikipedia.org/wiki/Wikipedia:Bildfr%C3%A5gor#Standardstorlek_p.C3.A5_minibild_.28thumbnail.29.