Hello,
When you are creating or editing your seller details, you can set the font and size by clicking on the buttons on the editor before you type in the information. If the information is already typed, highlight (select) what you want to change, and then click on the buttons to change it. Then save.
Sometimes the change doesn't "take" the first time, so do the highlight/select, change it to something else, click save, and then go back in and do it again, this time changing it to what you do want it to look like. Save again, and that should take care of it.
The font and size sometimes change automatically, if you are using one of Auctiva's templates. But all the font and sizes usually change consistantly and match when that happens.
Hope this helps.