You need an equals sign between the word "size" and the "+" (or the "-", or the number to get an absolute size). And drop the quotes.

You can get these right more easily by selecting the text you want to format and then selecting the formatting from either the icons above the text entry box or the pull-down menus above them. The Size pull-down menu doesn't include relative sizes, but you can easily enter a plus or minus sign after. All these icons and menus enter the formatting text into your message. so you can see what it is and follow the model later. Also, if you click "Reply w/quote" to a post that contains formatting, you can see its formatting code.