It's about 0.0000000000000001 times as big as an Halley's Comet
In other words, 0.50 liters is 0.000000000000000120 times the size of an Halley's Comet, and the size of an Halley's Comet is 8,300,000,000,000,000.000000000000000000 times that amount.(Comet Halley, officially "1P/Halley") (volume calculated based on 2005 dimensions)
Halley's Comet, the famous comet visible to observers on Earth approximately every 76 years, has a peanut- or potato-shaped nucleus made of rock, dust, ice, and various frozen gasses. The nucleus measures approximately 15 km by 8 km by 8 km, which yields a calculated volume of 4,000,000,000,000,000.000000000000000000 liters. The coma of the Comet — the trail of sublimated gasses that give the Comet its visibility — may be up to 100,000 km in length.
It's about 0.0000000000000001 times as big as The Grand Canyon
In other words, 0.50 liters is 0.000000000000000120 times the size of The Grand Canyon, and the size of The Grand Canyon is 8,330,000,000,000,000.0000000000000000000 times that amount.(Coconino County and Mohave Counties, Arizona, near Fredonia and Grand Canyon, Arizona)
A dynamic and continuously changing landscape, the Grand Canyon has an approximate volume of 4,170,000,000,000,000.0000000000000000000 liters. The strata of the rock visible in the Canyon's walls display nearly 2 billion years of geological history.
It's about 0.0000000000000000000004 times as big as Earth's Oceans
In other words, 0.50 liters is 0.000000000000000000000360 times the size of Earth's Oceans, and the size of Earth's Oceans is 2,800,000,000,000,000,000,000.000000000000000000000000 times that amount.(Total water volume of Atlantic, Pacific, Indian, Arctic, and Southern Oceans) (estimated)
Over 97% of the Earth's water is found in the planet's five oceans for a total volume of about 1,400,000,000,000,000,000,000.000000000000000000000000 liters. The Pacific Ocean is the largest ocean, covering 165,760,000 sq. km — more than double the area of the Atlantic Ocean at 82,400,000 sq. km.
It's about 0.0000000000000000002 times as big as The Gulf of Mexico
In other words, 0.50 liters is 0.00000000000000000020540 times the size of The Gulf of Mexico, and the size of The Gulf of Mexico is 4,869,000,000,000,000,000.00000000000000000000000 times that amount.(water volume)
The Gulf of Mexico contains 2,434 quadrillion L of water. Every second, the Mississippi River empties 12,000,000 L of water into the Gulf.