Use the Line Width edit box in the Style toolbar to define the width of lines in your selection, or to define the default line width for new objects. Line widths can only be applied to the solid line style.
The default line width is 0 or 'hairline'. When you print, a 0 width line will print as finely as the resolution of your printer will allow. On very high resolution printers, or when creating a high-resolution image, this may be too faint. In these cases you can increase the line width for all the objects in your drawing by a small amount (the amount will vary depending on the type and size of your drawing); the increase in line width should be barely noticeable on screen, but will create a much bolder impressions of lines in your printed output or high-res image file.
You can type your line width into the edit box or click on the spin controls to increase/decrease the line width by the X Nudge value set in the Dimensions tab of the Document Properties dialog.
