Hi,

Good question. This is from Solid Angle's Documentation about rendering hair objects: 

To add Arnold related parameters to the geometry you must add an Arnold tag to it.

There is the Min Pixel Width parameter. Just add the Arnold Tag to the Ox Hair object. 

Source: https://docs.arnoldrenderer.com/display/A5AFCUG/Hair+object

Jeordanis Figuereo (Product Designer. EPHERE Inc.)