I would like to change the image size in Effervescence+.

In the Administration page under Options for this theme, the field "Image size" cannot be changed. Is this a bug? Is there another way to change the image size?

Not a bug, but a theme decision. Some themes like this prefer to set a fixed image size for layout reasons. You will have to modify the theme (HTML/CSS) instead.

Thanks acrylian for this super-quick answer. I tried to modify the theme with HTML/CSS but did not manage yet. It seems that the option is well hidden.

You also need to change the image function calls. If you haven't you should read the theming tutorial.

Thanks!

I found it at last: the image size can be changed in themeoptions.php on line 42 and 59