When you open up a "reply" window, there are a bunch of icons right above the place where you type your text. Among them is a drop-down menu which says "sizes". You can drop it down to whichever size you prefer, and type in between the tags for size.
This is size 7 for example.
If you'd rather do a "quick reply" you can type in the size tags yourself - they are
[mumboSIZE="7"]This is size 7[/jumboSIZE] and take out the mumbo jumbo.
You can use sizes from 1 through 7.