To QuickBooks, the numbers you enter are “unit independent” — they can represent anything you want.
If you have exact measurements, enter the widths as inches, centimeters, or whatever unit of measure you used. If you don't have exact measurements, enter numbers that indicate the relative width of each column (for example, 10 for one column, 20 for a column you want to be twice as wide as the first, etc.).
You want the Item column to be .5 inch, the Description column to be 4 inches, the Quantity column to be 1 inch, the Rate column to be 1 inch, and the Amount column to be 1 inch. Since you have exact measurements, you enter them as follows:
Item .5(note the decimal point)
Description 4
Quantity 1
Rate 1
Amount 1
You don't have exact measurements, but you know that you want the Description column to take up 75% of the width, and you want the Item, Quantity, Rate, and Amount columns to divide up the remaining space equally. In this case, the easiest approach is to think of the entire form as having 100 units of width:
Item 8.25(1/4th of the remaining 25 units)
Description 75.00
Quantity 8.25
Rate 8.25
Amount 8.25