Change text size

To change the size of the text in the current buffer, use C-x C-- (i.e. control x then control minus) to decrease the size, and C-x C-+ or C-x C-= to increase the size. To reset to the default, use C-x C-0. These run the command text-scale-adjust which can take further input, so using C-x C-+ and then hitting + again will increase the size a second time, or hitting 0 will reset to default.

You can, of course, change the default text size and font. I add the following to my emacs config file:

;;use larger font
(setq default-frame-alist '((font . "Source Code Pro-14")))