Hello, how can i change the size of the generated Thumbnails in the Album (...index.php?album)? Options - Theme - Thumbnail Size hast no effet on any Theme. I find no way to set the size in any (config) file or so. Looks like a bug? Thank you.

NO, looks more like insufficient research. Also insufficient information. Please read our theming tutorial. Also look at the options.

For future reference, it is pretty useless to ask questions like this which are theme dependent without also including what theme you are using. It may well be that the particular theme does not allow you to make such a change.