As the system states..." Consider the width of the displays on which people read the wiki. Choose appropriate image and table widths."
If possible, the table preferences should be set to PERCENTAGE by default and not pixels. 100% is typical but based on system suggestions people should set their tables to 99%. Perhaps due to margin and padding differences.
This will ensure by default all tables comply with the "system statement." No extra effort on the user, unless they would rather work with pixels.