A tap to left or right of the volume circle will increase or decrease the volume by 1 click. 
 

Rather than sliding the slider, for fine volume adjustments I often use a kind of press-and-hold-the-button-and-roll-my-finger kind of action. You can watch the volume value change, or let your ears tell you if the volume is what you want. 
 

Or, there’s voice control: “Hey Sonos: volume up”.

I don’t know if that helps, pending anything from Sonos? 

Flip your controller to landscape (wide side to side) and the bar might get longer.

Tapping the ends of the bar will step the volume up and down. 

You should be able to use the controller's volume keys too.