I think the answer is that there is no normal height. So instead of trying to make wall-height placeables, release your textures as is, for use as splotches, or replacement textures for existing splotches. That way, people who just want a texture override for a scorch mark, can choose your texture when they recolor a tileset.
The cave set walls are usually concave or convex, so flat wall panels won't work there. The height of various dungeon sets differs by room.
Basically, having put a coat on Pstemarie's wall textures (or any other textures for that matter) only lets people use your placeable if that texture matches their walls in brick or stone per foot ratio. I'd find it hard to make a placeable like this which fits every, or even many, tileset walls.
That being said, your slime/blood/purple funk looks awesome and would make a great option as just a stand-alone texture. Or make a much smaller version of the slime for walls, and allow variants, that way people can choose to place flat panel ones adjacent to another and fill up the wall.
If a human height is less than 2m, I would make your largest one 2m. Some tilesets have a height change of 5m at one time, others only 2m in some places. Offering a wall-hanger type placeable, which can be adjusted in z-position by the area builder, seems your best bet. Go the decal route, allowing the user to overlay your object to any textured wall.
Again, great textures. I really like the second and third ones.
Edit:
I think there is a type of splotch in the OC cave/mine tileset which is supposed to represent fungus or slime on the wall. It also seems to have a self-illumination color set, because I can see it in total darkness. If your texture overrode or otherwise was used as a similar wall-hanger, that would make a great slimy cave. I could be wrong, and it is in the OC underdark set. I know it is a wall part in an open cave room, not a tunnel. I'll see if I can find which tile it is located on.
