Because you are going to use it primarily for receiving the antenna can be made from any size wire. Look into the cost for a reel of speaker wire which maybe less expensive than regular house wire, plus you get two wires for each length. That wire can be used for transmissions also up to 100 watts without any trouble because it usually is 18 gauge. Just pick a frequency about mid-band and use one of the online formulas to calculate for a half wave dipole. Spacing the wires about 30mm apart will be just fine if you eventually want to use it to transmit on, it will easily handle the power of the UBITX and a small amplifier. At the feed point just attach all the wires from one side of the attic together then to the coax center conductor and do the same for the other side except connect those to the shield and you will be good to go.