How to change the resolution after booting Arch Linux installation medium?

In addition to changing the font size to a larger one, with setfont, you can also pass kernel parameters on the boot loader line. The ISO uses Syslinux, so you can hit Tab when the menu appears and append these parameters to the kernel line. Two that would be most useful are:

• nomodeset to disable KMS
• video=1024x768 to force a specific resolution if KMS is required.

How to install and use a reasonable font:

1. Set up networking using a handy microscope
2. pacman --sync --refresh terminus-font
3. setfont /usr/share/consolefonts/ter-132n.psf.gz (UEFI mode) or setfont /usr/share/kbd/consolefonts/ter-132n.psf.gz (CSM mode)