Since a turret can cover one half of a sphere through elevation and rotation, and the 2D combat grid doesn't take into account the ship rotating along its long axis, a turret should be able to cover any firing arc. However, a ship with turret "dead zones" (where a firing arc is blocked by some other structure on the ship) could be an interesting handicap for a ship.
A battery of turrets may or may not be able to cover any arc, depending on the size of the battery, the positioning of the turrets, and the size and rotational speed of the ship.
I agree that Bay weapon arcs should be limited, and 2 or 3 hexes sounds about right...however, because of the axial rotation discussed earlier, the arc should be "mirrored" on both sides of the ship.
I agree with Capt. Jonah that a spinal should have a one-hex arc, either foreward or rear facing, chosen at the time of ship construction.