I am sorry I took so long.
The truth is I am not using deprecated font tags. Please try this:
<div style="font-size: 12pt">This is a three sentences paragraph. <span style="font-size: 18pt">The second sentence will be changed to a bigger font.</span> The third will remain using default size.</div>
Select the outer div and change the font size. The sentence in the middle will not change.