I have a HT with 600mm width bars, I want to replace with longer bars.

I intend on adding a harness etc on the front for kit, my question is….would there be a maximum lenght in the bars to use before (if) it had any effect on the bikes handling?

Looking at going 710/720mm.

Effect it how? Increase it by any length “feel different” compared to the 600’s.. stop worrying and get some wider bars fitted, then go try some silly wide ones and realise you should have bought some 750mm’s (or more) ages ago.

Steering will be a touch slower, your weight will be a tad further forward due to your arms being slightly further apart.

If needs must, both can be corrected by fitting a slightly shorter stem, would give it a few weeks with the longer bars before deciding whether shorter stem is necessary.