How do I change the font size on certain parts of a post on tumblr?

I created a page for my tumblr and its for one of those 30 day challenges. I want to make each day a bigger size font than the rest of the post but i don't know how to do that. Any help would be great.How do I change the font size on certain parts of a post on tumblr?
after you've typed your post, you click the button on the toolbar (above where you put all your text) that says ';html'; in small blue letters... then a window will pop up with the plain text version of your post



the text that you want to make larger, you put %26lt;big%26gt; in front of it, and then %26lt;/big%26gt; after it... the slash big is what signals the html to stop enlarging the text, so make sure you put that after the last word you want to make larger!



also, depending upon how large you want the text to be, you can add more %26lt;big%26gt; 's. but up to three is the limit, people find it obnoxious if its any bigger than that =p but you have to put as many %26lt;/big%26gt; 's at the end as you put before, for example:



%26lt;big%26gt;%26lt;big%26gt;%26lt;big%26gt; text here %26lt;/big%26gt;%26lt;/big%26gt;%26lt;/big%26gt;



also follow me if you like =) http://violetsarentblue.tumblr.com