The Common Wire Sizes are -
EU -
1.5mm²
2.5mm²
USA AWG Wire Gauge -
16ga (1.31mm²)
14ga (2.08mm²)
Though in your case, you might be able to go as low as 1.0mm² or 18ga (0.823mm²)
Here is what I would do in your situation, especially if you are not willing to take the speakers apart.
Get a terminal block like this -
25A 3 Positions Screw Terminal Strip Barrier Block
This is a THREE position, in your case, the middle connection would go unused. The gap of the middle unused screw connector separates the speaker wires and makes it much less likely for one wire to short out the other.
First attach the Barrier Strip to the back of the speaker. Then permanently connect the wires coming out of the speaker to the two outside connectors on the Barrier Strip. (Middle connection unused)
One of the wires should be marked, perhaps with a ridge or a stripe, be sure to locate the marked wire in the same position on both speakers. With a magic marker or a dab of paint or a paper label, marked the terminal strip position on both speakers that is attached to the marked wire.
Now, notice that the terminal screws are in pairs. You can attach external speaker wire to the other screw of the match pair of screws. You can use the marked wire as either Plus or Minus, as long as you do so
consistently on both speakers. That is, the marked wire is ALWAYS Plus or it is ALWAYS Minus on both speakers.
That allows you to use any external wire you want with the speakers.
Steve/bluewizard