http://websemantics.co.uk/resources/...version_chart/
That page is very good for showing relative sizes. Also, if you prefer to use ems instead iof the keywords, it will show you what ems correspond to what keywords.
http://pxtoem.com/
This is a px > em or em > px converter. This is good for using when you have the size you want in px but know you should be using em instead.