What gauge ground wire do I need for a 220V oven?

#10 copper is large enough for up to 60 amps for an equipment grounding conductor according to NEC 250.122, but I would use #8.

But Steven is right about legally installed existing 3 wire circuits being allowed (see below), normally the installation instructions show how to make the ground neutral bond inside the appliance when changing to a 3 wire connection.

250.140 Frames of Ranges and Clothes Dryers. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be connected to the equipment grounding conductor in the manner specified by 250.134 or 250.138.

Exception: For existing branch-circuit installations only where an equipment grounding conductor is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be connected to the grounded circuit conductor if all the following conditions are met.

(1) The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.

(2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.

(3) The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.

(4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.

(NOTE: The "Grounded Conductor" is NEC terminology for the neutral wire.)

The proper gauge would be the same as the current wiring. It is preferred to replace the entire cable instead of merely adding a ground wire.

Note: While code most likely requires a 4 prong outlet for NEW circuits, it often allows switching the cord from the appliance to 3 prong if you already have a 3 prong outlet that was legal when installed. This requires bonding the neutral and ground inside of the appliance, but someone in a decent hardware store can teach you how to do that and sell you the required cord.

same size [gauge] as the existing neutral -- which should be same as either of the hot lines