The line tool is not affected by the brush size selected. Set the line Width in the Options Palette. When drawing a line the Info Palette will give you valuable information. Normal is the default for all tools with blending modes. Other modes may be selected as needed.
Size is measured in pixels.
I always leave this box checked. Check the appropriate box. Click this box to get the arrowhead shape dialog box. These values are based on the current line width. Negative values result in a diamond shape concave arrowhead.