
It's about as big as a Postage Stamp.
(average; United States dimensions)
A typical postage stamp measures an average of 400 sq. mm. One of the most prized stamps among collectors is the USissued "Inverted Jenny" stamp printed in 1918, which features an upsidedown image of Curtiss JN4 ("Jenny") airplane; a single stamp sold for $977,500 at a 2007 auction.
It's about oneandonetenth times as big as a Nickel.
In other words, 400 square millimeters is 1.1320 times the size of a Nickel, and the size of a Nickel is 0.88340 times that amount.
(United States fivecent 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 353.3 sq. mm.
It's about ninetenths as big as a Quarter.
In other words, 400 square millimeters is 0.86540 times the size of a Quarter, and the size of a Quarter is 1.1560 times that amount.
(United States quarterdollar 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 462.2 sq. mm.
It's about oneandtwofifths times as big as a Penny.
In other words, the size of a Penny is 0.7140 times 400 square millimeters.
(United States onecent coin) (a.k.a. Lincoln penny, a.k.a. Cent, a.k.a. One cent piece)The Lincoln penny, in its current design since 1909, has a diameter of 19.05 mm (0.75 in) and an area of 285.0 sq. mm.
It's about oneandthreefifths times as big as a Dime.
In other words, 400 square millimeters is 1.5880 times the size of a Dime, and the size of a Dime is 0.62970 times that amount.
(United States tencent coin) (a.k.a. Roosevelt dime)The dime, in its current design since 1946, has a diameter of 17.91 mm (0.705 in) and an area of 251.9 sq. mm.
It's about half as big as a Secure Digital (SD) card.
In other words, 400 square millimeters 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 770 square millimeters. 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, 400 square millimeters is 5.1450 times the size of a Nailhead, and the size of a Nailhead is 0.19440 times that amount.
(for 11gauge wire nai)An 11gauge nailhead measures 77.740 square millimeters. Nails have been used since ancient times; archaeologists have discovered Ancient Egyptian nails that are over 5,000 years old. 
