How to increase the line thickness of linear takeoff?

Change measurement type to one of the linear with width options, enter a width, check "Scaled", and select a side of line.

I can increase only opacity of the line, but not the width of it, is that right? I'm using only Linear , not the Linear with width.

As far as I know, yes, that's right for an item that is set up as only linear. You have to switch it to a measurement type with width to be able to adjust the width.