To my surprise, in Windows 10, there is no control for the scroll bar width (at least not that I can find). In Windows 7, it was directly controlled by one of the Advanced screens in the control panel. I would have thought it would be a prime contender for an entry in Windows 10’s Ease of Access controls, but no.
So a bit of Googling, and the answer is to change it in Regedit. I think it is poor that the setting is not more accessible.
I now have ScrollHeight and ScrollWidth set to -400. This is totally over the top, but I like it. The default is -225, so I reckon -300 is probably a more subtle widening.
Note: not all programmes use the Windows settings unfortunately. Poor show, Slack!