Command and Plus (+) — that’s it. Its that simple, stupid!
And I was fidgeting with all kinds of things in the ‘Preferences..’
Fortunately, a simple google search saved me a lot of time.
Well, it seems that command and plus (+) only maintains the font for that session. If you want the font size changes to stay permanent, then you get into Terminal->Preferences->Profiles and look for ‘Font’, as shown below. For me Font Size 15 seems to work.