The short answer is that you cannot. Everything within the same text object will have the same size. So if you want to somehow "customize" the size of individual items you will have to use for example additional text objects with smaller font sizes (or extensions, of course).
No, since settings like font and size apply to expression output as a whole.
Your question is sparse on details of the context. IS your expression used in a text box? In a Pivot/Straight table? In a caption?
In a Straight table, you could define a second column with fixed content (just a %) that is left aliigned. Text properties for the '%'-column can then be tuned using the Custom Format Cell-dialog (i.e. text size can be reduced to 80%) You can remove the cell border between the two as well.