What's the suitable wire size for 9VDC @25 Amps line?
You have three things to worry about: heat, voltage drop, and interference.

Interference between your DC and RF wires is probably the least of your problems, unless your high-current lines dump noise onto the RF lines, or the RF lines are really powerful, and dump enough energy into the DC lines to cause problems for whatever you're powering.

For the heat problem, you can probably size the wires as if they were in a car.  Find a table of recommended wire sizes, and maybe do a test to see if the wires get hot.

But -- voltage drop is going to be your enemy.  Just nine volts implies that you're going to need to restrict the voltage drop to 1/4 or 1/2V (or less -- you know what you can stand).  The general way to deal with this is to calculate the maximum resistance that you can stand (i.e., 0.25V / 25A = 0.01\$\Omega\$), look up the resistance per meter (or foot, or whatever) of various sizes of copper wire, then choose a wire diameter that will result in the entire length of your wire run (there and back) to have less resistance than your specified amount.

I suspect that once you get wire that fat, you'll be well above the size you need to be safe from heating.