I need the size of the gap in a spark plug to a 2004 Hyundai Elantra

You won't need to change the gap. When you buy spark plugs for most modern cars the plugs are usually preset to the correct gap. It is usually only adjusted if you are changing the performance level of the car.

Spark plug manufacture sites & the internet

Any plug you buy has the gap on the web site

wires are mixed up or you have the spark plug gap wrong. remove number 3 and one spark plug and compare the tips

Spark gap is the gap between the electrode and the arm that is needed to a specific size per vehicle and engine size. If this gap isn't correct, you may have problems with missing in the engine. To find out what the gap size is for your vehicle, you can go to your local auto parts store like Autozone and either buy the repair manual for your vehicle or call them and ask them what it should be; they can always look up that information in their systems.

is on the side of your oil pan

Go to your local parts store, let them know what you want, for year, model, engine size. Ask them what is the gap on the plugs. You will need a spark plug gap gauge, a spark plug socket, 6" extension and a ratchet. Remove your plug wires one at a time and replace the plugs one at a time. This keeps all confusion to a minimal so the wire don't get crossed. The plugs are on top of the engine down through the valve cover. You want a socket with a rubber spark plug boot in it.

it is either 13/16 or 9/16

Inspect #2 spark plug and the ignition wire to it. Inspect the connector to the plug and to the coil... replace as needed.

.044 is the gap size you need

Under the hood you will find a white sticker, about 2-3 inches wide. On it will have the engine size/group and the spark plug gap and other service information. Good luck