All wires and cables are sold according to their section and it is stated in mm2 or AWG on the wire itself
Here is simple mm2(area) to mm(diameter) conversion valid for solid wires.
Wires containing multiple strings may be same effective conduction area, but to measure more in diameter, because the air between the strings.

section = effective diameter - max opening to mount the wire
6mm2 = 2.8mm - opening is 4mm
10mm2 = 3.6mm - opening is 4.6mm
16mm2 = 4.5mm - opening is 5.5mm
25mm2 = 5.6mm - opening is 6.8mm
35mm2 = 6.7mm - opening is 8.8mm
50mm2 = 8.0mm - opening is 9.8mm
70mm2 = 9.5mm - opening is 12mm