No - this is the default setting. That means that whatever a user enters (without selecting a font size from the fontSize select list) will appear on your page or email template using the style that is applied to its containing element wherever the HTMLEditor text is output to. This ensures that - unless otherwise specified - the style that is applied will be the correct one as defined by you in a stylesheet or inline style for that containing element.
I think the best way for you would be to make sure that the font-size is specified in the css for the containing element (i.e. in the div or span on the page or template that 'contains' the output from whatever is entered in to the editor.
If you set a default text size for the editor, then everything that the editor window contains will change to that default, which on some lines will be OK, but in others it won't; in some cases you might end up spending more time having to set a style for much of the input text that you don't want to appear in that default style - especially if you have instances of the editor on other pages, now or in the future.
HTMLEditor also allows you to attach your own stylesheet (the little pencil to the right of the Editor Preset Drop down list > 'Styling' tab) in which you can specify the body style it will apply to default input, - this may be what you're after, but the safest way to get the end results in specific places is to set the style for the containg element on the page itself. I think.