There are simple and more complex calculators available online. This Polk Audio one is simple but I think a useful baseline or "sanity check":
support.polkaudio.com
Using the stricter scenario - a 4 ohm speaker load as opposed an 8 ohm speaker load - the Polk chart says you can use 18 gauge wire for up to a 16-foot run.
Polk doesn't list 19 or 20 gauge (which is what 0.75mm wire would be), but extrapolating from the chart's other entries it seems likely they would say you can go up to 10 or 12 feet with 19 or 20 gauge wire.
With that said, a lot of folks - including me, and I would guess including a lot of other ASR members who believe strongly in the measurement-base approach here - nonetheless prefer a little bit of "overkill" or "wiggle room" with their speaker wires because it just makes us feel better.
Are you specifically looking for brown
fabric covered wire? If you are okay with the look of brown plastic insulated wire, there are options available to you in lower (thicker) gauges.