Updated June 15th, 2019.
Several Mac apps let you enlarge text and pictures via ⌘+. Sometimes it’s ⌘ and Shift and +, so try that too. The “+” should be the one above the = key, though the one on the numeric keypad might work. Apps where this works include web browsers, Mail, Microsoft Word, Pages, Messages, and probably more.
You’ve probably guessed that ⌘- (Command-minus) makes things smaller.