Hi wengles!
By default Document Gallery instance spreads all over the width of the given parent block and centers (horizontally) its content along the occupied area. It seems that in Your case something has gone wrong and unfortunately there is not enough info to figure out how to fix it (problem could be in CSS of custom theme, if You use one, or any installed plugin, also it could be some typo in shortcode or anything beyond mentioned).
To better assist You we’d prefer to see what You get, so, please, if You can, give us a link to the problem page.
To change font size You have to set font-size (by default it’s only 10 pixels).
Given below CSS should be pasted in the Custom CSS textbox at Dashboard -> Settings -> Document Gallery:
.document-gallery .document-icon a {
font-size: 14px !important;
line-height: normal !important;
}
Feel free to adjust values so the result meets Your needs.
Hope this will do the job for You.
