To know which resolutions are supported by your screen, install hwinfo:
# pacman -S hwinfo
# hwinfo --vbe | grep 'Mode '
Modes are displayed in hexadecimal.
/boot/syslinux/syslinux.cfg and add to the
APPEND line the desired
<resolution> is the corresponding hexa code.
For example: add
vga=0x037f if you want 1920x1080x24.