I used the resistance value you had. Even though it is in the metric system, I converted it into the equivalent 1000 ft that is in standard tables in the US that has the gage values.
If the looks matter and if you can handle that 6mm, use it.
What is the next lower size available to you, 5mm or 4mm? Need resistance to recalculate
Looking in my chart, I think your size is for the cable with insulation, not the copper wire
size.
Ive been able to check the 2.5mm cable and yes it is the internal cable with insulation.
Next size down is 4mm
description Van Damme Blue Series Studio Grade classic pro loudspeaker cable 2x4.00mm twin axial
Overall diameter mm 11
Weight Kg/km 162
Conductor stranding mm 227 x 0.15
Conductor resistance Ohm/km 4.6
I'm thinking the 6mm cable is designed for an amp with XLR connectors. My amp is a low budget here is the spec.
Amplifier
Power Output
Front L/R 130 W/Ch
(6 Ω, 1 kHz, 1%, 1 channel driven, IEC)
Center 130 W
(6 Ω, 1 kHz, 1%, 1 channel driven, IEC)
Surround L/R 130 W/Ch
(6 Ω, 1 kHz, 1%, 1 channel driven, IEC)
Dynamic Power 180 W (3 Ω, Front)
160 W (4 Ω, Front)
100 W (8 Ω, Front)
My Speakers are bookshelf with 100 W/Ch (i don't know if this changes
anything)