The font sizes for the tag cloud are set programmatically according to how many tagged posts for each tag exist in the blog. You can update the overall font size, however, that would defeat the purpose of the tag cloud by making all of the links the same size. Here is an example:
.wp_widget_tag_cloud a {
font-size: 12pt !important;
}
Note that if the blog is new, the fonts may all look really small because not enough posts are tagged yet. The font size for the tags used more often will get bigger and start to stand out automatically—which is the purpose of the tag cloud widget.
In comparison, the category cloud widget has additional settings for "Minimum font percentage" and "Maximum font percentage" which give you a bit more control over the look if using a category cloud instead of a tag cloud interests them.