You can change styles such as font sizes by adding some CSS rules to overwrite the styles of your theme, which is where the current styles are set.
When you set up a CRED form you will see that you have the option to add custom CSS rules (expand the section CSS and JavaScript for this form).
If you don't know much CSS you will need to do a little studying. There are lots of online tutorials, but here is a good place to start: hidden link
In terms of what elements you need to style, the easiest way to see what rules are currently being applied which determine their style is to use the developer tools which most modern browsers have.
Normally you can right-click on something you want to customise, e.g. a title in your form, and choose "Inspect" and that should bring up the dev tool panel that shows the markup and the styles being applied to it. See the screenshot for what that looks like in Chrome when I inspect your name on this page, for example.
You need to remember when thinking about styles that they cascade (CSS = Cascading Style Sheets), i.e. you may not see the font-size explicitly set on an element because it is being inherited from a parent element. You might want to change the style on the parent (which will affect all children that inherit from it) or just set the style on the element itself to affect just that element.
