It's about as big as a Postage Stamp.
(average; United States dimensions)
A typical postage stamp measures an average of 0.000000400 dunams. One of the most prized stamps among collectors is the US-issued "Inverted Jenny" stamp printed in 1918, which features an upside-down image of Curtiss JN-4 ("Jenny") airplane; a single stamp sold for $977,500 at a 2007 auction.
It's about one-and-one-tenth times as big as a Nickel.
In other words, 0.000000400 dunams is 1.1320 times the size of a Nickel, and the size of a Nickel is 0.88340 times that amount.(United States five-cent coin) (a.k.a. Jefferson nickel)
The nickel, in its current design since 1938, has a diameter of 21.21 mm (0.835 in) and an area of 0.00000035330 dunams.
It's about nine-tenths as big as a Quarter.
In other words, 0.000000400 dunams is 0.86540 times the size of a Quarter, and the size of a Quarter is 1.1560 times that amount.(United States quarter-dollar coin) (a.k.a. Quarter dollar, a.k.a. Washington quarter, a.k.a. two bits)
The George Washington quarter, in circulation in various forms since 1932, has a diameter of 24.26 mm (0.955 in) and an area of 0.00000046220 dunams.
It's about one-and-two-fifths times as big as a Penny.
It's about one-and-three-fifths times as big as a Dime.
It's about half as big as a Secure Digital (SD) card.
In other words, 0.000000400 dunams is 0.520 times the size of a Secure Digital (SD) card, and the size of a Secure Digital (SD) card is 1.90 times that amount.A Secure Digital (SD) card measures 3.2 cm by 2.4 cm for a total area of 0.000000770 dunams. SDXC Cards, the latest standard for the technology, will allow the cards to hold up to 2 terabytes of data.
It's about five times as big as a Nailhead.
In other words, 0.000000400 dunams is 5.1450 times the size of a Nailhead, and the size of a Nailhead is 0.19440 times that amount.(for 11-gauge wire nai)
An 11-gauge nailhead measures 0.000000077740 dunams. Nails have been used since ancient times; archaeologists have discovered Ancient Egyptian nails that are over 5,000 years old.