Voltage drop when load is applied
A 24V, 60W bulb requires 2.5A of current. 25 such bulbs requires a total of 62.5A.

If you want to deliver this current with a voltage drop of no more than 10% (2.4V), then you need to use wires that have a total resistance of no more than 2.4V / 62.5A = 38.4 m&Omega;.

Assuming you're using copper wires with a total length of 40 meters (out and back), they would have to have a diameter of about 5mm, or roughly AWG4 &mdash; i.e., welding cable.