I have a small crack on the side of my number six cylinder. Can that cause my truck to lose compression

If your block is cracked you need a new engine.

If it is indeed cracked, then it may only be a matter of time before crack reaches the water jacket. Any cracks in the cylinder, like scratches, can rob you of compression, you might also expect some oil blow by, unless crack is below head of piston. No matter what, your block should be replaced or repaired; they might be able to sleeve the cylinder for instance but that will mean pulling motor and rebuilding it with machining involved.

Did you run straight water in the engine and it froze?