Possible Clues:

• Newspaper ad meas
• Newspaper ad meas.
• Newspaper space meas.
• Small area meas.
• Tiny fraction of a 21 Across: Abbr.
• 1/64 of a checkerboard, maybe: Abbr.
• Certain area meas.
• Area meas.
• Two-dimensional meas.
• 6,272,640th of an A.