Highlight the text to be changed. Select an appropriate style, heading or size from the combo box in the toolbar.
While you can alter font size only (using one of the 'size' options) and perhaps set it as bold, this will handicap your future editing. If you instead use for example "Heading Medium" for most headings and "Heading medium" for sub-headings it will achieve the same imediate desired effect. Note that "Heading Large" is reserved for the first heading on the page, and this is normally managed in the RENAME area of each page.
The advantage you will have, is that when in the future your site is being re-styled all headings will be able to be changed globally (using a stylesheet or in the global design settings area for your whole website). For example all "Heading Large" could be changed to a certain font and colour and be underlined and bold, by a very small simple edit that would apply to your whole site - no matter how many pages.
Using the standard headings as mentioned above, will also help your SEO, and also help visually impaired people navigate your site more effectively.