To determine wood screw size, measure the thickness and width of the wood that the screw is entering and root diameter of the screw. Ensure that the size of the screw is such that there is enough wood material beyond and around the length and diameter of the screw.
Ideally, the screw should be sized to pass completely through the first wood piece and must not poke out through the second piece of wood, as this may cause the wood to split. There should be at least 1/4 inch or more of wood remaining beyond the end of the screw.
Additionally, the wood screw should be of a size that leaves at least 1/8 inch of the wood material on either side of the root diameter of the screw. Root diameter refers to the width of the threaded part of the screw, and usually varies between 1/4 and 3/64 inches.
Also, since oval and flat-headed screws are countersunk below the wood surface, the head length must also be considered when measuring for the screw size. Countersinking is not needed for screws with round heads. Finally, be sure to double-check all the measurements taken.