There is no native, declarative way to increase the size of fields on a page nor a way to inject your custom CSS into a standard page layout.
You can however create a Visualforce page and use the apex:detail component to show the entire detail of the page and optionally, the related lists. This allows you to surface a page, as defined by the page layout and permissions associated that that user's profile, but through Visualforce. Since it is a Visualforce page, you can include your own CSS that overrides the standard CSS. What I would advise you here though, is that you're ranging into "hacking the page" territory. What Salesforce chooses to call elements and selectors on the page in not guaranteed to stay the same over time so use this method at your own risk.
There are other methods, that I'm less familiar with, like using sidebar components.