If you mean the bbcode for changing sizes (or what sizes are available to you) then you can simply play around with:
[size=Npx][/size] or [size=Npt][/size]
where
N can be used as a modifier for the size. An example of the size bbcode on size 30pt was used in this post. You can quote and see the behind-the-scenes to get a better understanding of how it works.