Much will depend on the "C" rating. If your battery is 20C then it will put out 100a. Current flow creates heat, heat damages insulation be it the wire or connector. If you base your connector size on the "C" rating you can't go wrong.
"XT60"s may not tolerate that kind of current for very long until they start breaking down. If the connection shorts you got a problem!
Wire size is also important. You can go larger but never smaller. Smaller wire will act like a resistor increasing current and heat. Just remember that wire gauge is backwards, the smaller the number the larger gauge it is. For example; 12 is larger than 14, 6 is larger than 8 and so on.
Build your power system to withstand the max current your battery can put out which would include the ESC. The motor will draw what it needs from the "system", be sure it is up to the task