How do I change the font size in Dwarf-fortress?

You have two options: zoom in or use a larger font (or tileset).

To zoom, you can use the mousewheel or the [ and ] keys to zoom in and out, which is good for quickly getting a close up look at something or a quick overview of your domain. It does tend to make things blurry, though. The ZOOM_SPEED setting affects how fast it zooms.

To use a different font, you'll have to edit data/init/init.txt (which is also where you find ZOOM_SPEED). There are a lot of settings here, most of them well documented in the file itself, but the ones you're interested in are FONT, GRAPHICS_FONT, and GRAPHICS_FULLFONT, which control the font in non-tiled, tiled windowed, and tiled fullscreen modes.

The default font is curses_640x300, which is intended for use in an 80x24 tile game window at 640x300 resolution (hence the name), so the tiles are pretty small - 8x12. curses_square_16x16 is a better choice; the tiles are square, and noticeably larger. If you want something different (or larger still), you can download a wide variety of fonts and tilesets from the wiki.