You can install the package while using the live-dvd. You need to add non-free to the sources.list.d/debian.list. Then do apt-get update && apt-get install firmware-linux-nonfree
. If you install right then, the firmware will be in your installation too. You could also try cli-installer, which might be usable in your situation. You could also try to set dpi according to your screen size with the xrandr command.
greetz
devil