How to set two different font sizes in the same <p>

<p id="losinfo">Los Santos: <span id="secondText"> a sprawling sun-soaked ...</span></p>

Then style #losinfo and #secondText differently in css.

Your question is a bit unclear.
Are you meaning something like this?

<p id="losinfo">
<span style="font-size:20px;">Los Santos</span>
<span style="font-size:15px;">: a sprawling sun-soaked metropolis [...]</span>
</p>

Use a div around the part you want 20px and another div around the part you want 15px is what I would do. Then set those divs with classes and set the font-size in css.