Smaller font of certain widget and publication spacing issue

Also when I open a publication there is this annoyng white space betwen the main menu and the title. I tried to determine with firebug how to remove it but I was unable to find the line in css.

The space is a combination of bottom padding in the nav items and top padding in the article.

You should create a Child theme to make your changes. If you do not, all of your changes will be lost if you update your theme.

Try adding this to the end of your child theme’s style.css:

#content nav { padding: 0 !important; }
.singular .hentry { padding: 0 !important; }

I added the latest post widget to my sidebar, but I want the titles only in this widget to be smaller than the rest of the links in the sidebar

Since I can’t identify the links (I only read English), I can’t be sure which links you want to be smaller.

Also I want to add a widget to the publications template sidebar.

Is the problem that you don’t know which sidebar, or that you only want the widget to appear on page(s) that use that template? If the latter, the Widget Logic plugin might help.