I'm sure there are freeware modelers with measuring tools. There are tons of freeware modeling tools in general. That said I've never looked for this feature specifically so I don't know which ones have it. Make sure to check all the usual suspects: Blender, anim8or, Wings, OpenFX, Truespace (older versions are free,) etc.
Do note however that using realworld measurements in a game presents a lot of problems. Experienced level designers know that paradoxically locations look more "real" when distorted in a number of important ways than when built to realworld measurements. Among other things there just generally needs to be more open space for most locations. These level design principles are very similar to set design; people who do film and TV know that a room which feels roomy in person suddenly seems cramped and cluttered when seen through a camera and without the benefit of peripheral vision.
Do note however that using realworld measurements in a game presents a lot of problems. Experienced level designers know that paradoxically locations look more "real" when distorted in a number of important ways than when built to realworld measurements. Among other things there just generally needs to be more open space for most locations. These level design principles are very similar to set design; people who do film and TV know that a room which feels roomy in person suddenly seems cramped and cluttered when seen through a camera and without the benefit of peripheral vision.