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

I now have this code in Headline box: Links (and I saved changes)
color: #016563;
text-decoration: none;font-size: 175%;

The new color (dark green) and font size aren't showing up. I've tried looking at it from another computer, too.

I have another blog that also uses Atahualpa and it has much larger titles, but my code there for links is
color: #7CB8B8;
text-decoration: none;

What am I doing wrong?

what is the url?

http://www.ellenfinkelstein.com/acadblog/

One of your problems is you have a caching plug-in active. This can cause you to make a change but still see the old page that is in the cache. You should deactivate the plug-in while you are doing changes to the style of your site.

That helped for everything except HEADLINE Box: Text, which is exactly the same as HEADLINE BOX: Links, that is:

color: #016563;
text-decoration: none;font-size: 150%;

My individual posts pick up the color, but not the font size. Yet on my blog's home page, it's picking up the larger size.

Instead of % use px

That worked, thanks.
I'm going to lose these settings when I upgrade manually, as recommended, right?
I exported the settings, but they aren't exactly easy to read. Any best practices for dealing with this issue?

With Atahualpa any settings you make in the theme options, even the CSS Inserts, are stored in the database and will exist after an upgrade that's one of the things that makes this such a great theme.

Now if you edit any of teh theme code, those will go away and you will have to redo them.