change font size of existing letter
Have you tried highlighting all the text you want to change, then using the Font tool to change the font to whichever font and whatever size you want? (This is the standard procedure in every word processor that I have ever used.)
Click inside your body text, open the Style sidebar F11, identify the paragraph style that is highlighted. Right-click on the style, select Modify… Change the size and OK out.
If you have direct formatted your words then the style will be overwritten by the direct formatting (can be cleared by selecting the text and pressing Ctrl+M). As a letter is normally only a page or two then it might not be a problem to apply direct formatting as suggested by @ve3oat . For a longer document than that it would be a mistake to use direct formatting. Cheers, Al