This represents a large portion of one of the rooms in Putrefier. It looks quite robust in game and takes up a mere 212kb uncompressed. Not a high price to pay for a lot of aesthetic stimuli. It was constructed in Photoshop using existing textures in the resource wad, some splatter brushes and a bit of artistic ingenuity. GZDoombuilder makes it simple to align your floor / ceiling textures by dragging them into place. I should note that the larger the mega-texture, the more you'll run a risk of encountering a more pronounced blurring effect and a strain on the images' color depth. If need be, create your master texture and break it up into smaller pieces to avoid these undesired effects.
Another thing to keep in mind is if you have a general idea of what kind of lighting your room is going to have, adding additional bits of shadow and/or highlights is a nice touch. You're already using the real-estate, might as well make the most of it.
This method also works well for creating more realistic outdoor environments as well.
Outdoor areas, depending on the desired level of detail, can be strung together in Photoshop using 3-4 base ground textures with various rocks and debris peppered in. Clone stamp is your best friend.
This level of detail, albeit a low definition rendition, is something that hasn't been seen outside of more modern games. As heavy handed as it may seem, given the small file size and quick construction time, the end result could put an area of your map completely over the top for a relatively cheap price tag.