To increase OVERALL size of Dungeons of Dredmor:
1) download virtual box https://www.virtualbox.org/wiki/Downloads and install new guest system
2) in VB menu: view -> scale factor -> 150%
3) assign the same low resolution as your desktop has to the game
To make font bigger and better readable:
1) download new fonts /games/Dungeons-of-Dredmor/big_size_font.7z (thanks to Ramza for increasing fonts!)
2) replace fonts in there: <your_disk_glyph>:\Steam\steamapps\common\Dungeons of Dredmor\fonts
Video guide instructions: