To set thumbnail size to different value than the default
 

I would like to get similar thumbnail size (like on imagetwist.com, imagebam.com or picstate.com) the longest side of the thumbnail picture is set to 350 px. For me this seems to be optimal. However setting different values eg. 400 or 450 would be also nice and welcome.
The default "small" thumbnail is too small for me, the "medium" is too large.