Since I upgraded to the latest version of Atahualpa, my post titles got smaller. (Maybe I had some setting that was lost.) They are now smaller than my Heading 2 headings that I use throughout my posts. Is there some easy way to make them all larger?

Look in Style Posts & Pages

I did look there but didn't see any code relating to the size of the text, just margins, color & padding. Where is the current size being picked up from? I didn't see anything in the CSS editor either.

you didn't see anything about headlines? what version of Atahualpa are you running?

I'm using 3.7.3. I see Headline Box: Text and Headline Box: Links.
But the only code there is:
padding: 0;
margin: 0;

and

color: #666666;
text-decoration: none;

respectively.

I still don't know how to change the size of the text. Do I need to manually create some CSS code?
If I do, what type of text are the headlines? h1? h2? I don't know.

you can put ANY css in that box that you want to effect the header. Add 'color: #00ffff;' and you will get a pink, so stet thhe font size you want there