Semantics is all about the intended meaning of the content you're wishing to mark up. It has nothing to do with style. big is a visual attribute, therefore it's a style, so has no place in your html. <b> is also style, so should not be used. If your wish to emphasise a portion of text, you will wrap it in <em> if you wish to strongly emphasise text, you'll wrap it in <strong>. The best thing you can do is read the intended uses of those tag's that you're unsure about at W3Cschools is a good place to start.
Using Microformats is a matter of personal choice and one which I advocate, but the alternative is not <address>. As I mentioned in another post, the <address> element in HTML is NOT for marking up your street address in a contact section.