For some reason I have a hard time remembering this. If you have a hard time too, consider this my Christmas gift to you.
#Emacs of the day: Demoing code to at your desk? C-x C-+ to increase font size, repeat C-+ and C-- to inc/dec font size, C-x C-0 to reset
— Robin Green (@fatlimey) August 31, 2015
Although Green doesn't mention it, you can skip the 【Ctrl+x】 after the first invocation. So 【Ctrl+x Ctrl++ Ctrl++】 will increase the text size two jumps.