I don’t know what gray box you are talking about since there are a lot of different themes out there and they all look different, but I’ll assume that you are talking about what I call the “post summary” at the bottom of the post in the Kubrick Theme.
The way to learn how to do this is to look at a test post in your web browser. From the menu choose VIEW > PAGE SOURCE. It will pop up a page with the generated HTML version of your page. Go through the code and look for the different ID and CLASS names for the different sections on the page, looking specifically for your “post summary” section that says “This entry was posted…”. It will have something like:
<p class="postmetadata alt">
<small>This entry was posted on Tuesday....
Then go through the style.css file in the wp-content/themes/default/ folder (or change default to whatever folder your theme is in) and find the style.css file. Open it by double clicking on it and scroll through the file to find “postmetadata”, the style “selector” (name) for that section. You can make font size changes there. BUT notice that there is a tag that says “small”. If you delete that, and it’s matching closing tag, then your font size will be larger.
To learn more about CSS and WordPress:
WordPress CSS Information and Techniques
WordPress Blog Design and Layout
Using WordPress Themes
