The settings don't provide a way to do this, so you'll have to edit one of the skin's template files. This means that the change will apply to any album you create with Galleria (unless you change the file back, of course).
The first step is to create a copy of the skin, so that your changes won't be overwritten by a new installation of jAlbum. Choose Tools, Skin Developer, Create new skin, give the skin a new name, like MyGalleria, and choose Galleria as the basis for the new skin. Then, when it asks if you want to edit this new skin, say Yes. (If you want to make more changes later, you can choose the MyGalleria skin in the main jAlbum settings, then Tools, Skin Developer, Edit skin files ).
In the editor that opens, choose File, Open, open the res folder, and open the index.css file for editing. On line 46, you can change the size to whatever you like. To change the style, you'll have to add new attributes. For example: