In general, the powder chamber needs to be big enough to hold the right amount of powder. A little larger simply means a little air space - but because of the way it's built you'll have the same amount of air space so it works well.
An inch in diameter could be small - depends on how deep you cut it 1" diameter x 1" deep WILL be too small. That's an oppinion based on a 2.67" (old beer can - current soup can caliber) of bore length of 1/4" + one beer can height - filled with sand/water/and/or concrete). A longer tube uses less powder (longer push time).
The smaller in diameter and longer in length the more steel you'll have around it (holding OD constant). How pressure is affected by having a longer or shorter, larger or smaller diameter - I do not know, I would be guessing.
I have one in 24oz soda-pop-bottle caliber that uses much less powder for a given weight (and less noise) because the bore is about 18" long.
