Your best option then is probably to use a tool such as firebug, or chrome developer tools to identify the selectors you want to change - you can even use those tools to experiment with the changes to get the look you want, before adding those changes into the theme customcss setting. Most themes over the last few years, including Clean, have such a setting and its the easiest way to customise your theme if only css changes are needed (ie no changes to the layout files etc).
Use the customcss setting rather than manually altering a css file, so that your changes will persist through any future upgrades you do. The alternative would be to create a child theme with your changes, but if you are only making a few small relatively minor changes, then I would recommend the customcss setting rather than creating and maintaining another theme.
If you believe there is a bug in the design (such as a 'first line indent') you may want to propose a change through the tracker - although the 'font is too big' is probably a subjective opinion rather than a bug as, although quite a number of people do ask for them to be made smaller, I have in the past had a number of people asking for them to be larger too
(and on occasion, different people involved with the design of the same site!
)
Richard