Can I Have Multiple Sizes Of Text In A Single Text Box?

You certainly can.  A single text box may include as many different typestyles and type sizes as you wish. To change typestyles and type sizes of text that is already contained in a text box, just highlight the text that you want to change and adjust it however you see fit.  The only words that will change are the ones that are highlighted. You can also select your typestyle and type size preferences before you begin typing so they take on the desired attributes as you type.