Jupiter Red Spot_1
In this comparison image the photo at the top was taken by the Hubble Space Telescope in 1995 and shows the spot at a diameter of just under 13,050 miles (21,000 km); the second down shows a 2009 photo of the spot at a diameter of just under 11,180 miles (18,000 km); and the lowest shows the newest image from taken in 2014 with the spot at its smallest yet, with diameter of just 9,940 miles (16,000 km).
Earlier this year we reported that amateur astronomers had observed and photographed the recent shrinking of Jupiter's iconic Great Red Spot. Now, astronomers using the Hubble Space Telescope concur:

"Recent Hubble Space Telescope observations confirm that the spot is now just under 10,250 miles (16,500 km) across, the smallest diameter we've ever measured," said Amy Simon of NASA's Goddard Space Flight Center in Maryland, USA.

Using historic sketches and photos from the late 1800s, astronomers determined the spot's diameter then at 25,475 miles (41,000 km) across. Even the smallest telescope would have shown it as a huge red hot dog. Amateur observations starting in 2012 revealed a noticeable increase in the spot's shrinkage rate.
Jupiter Red Spot_2
© Etienne Trouvelot
Drawing of Jupiter made on Nov. 1, 1880 by French artist and astronomer Etienne Trouvelot showing transiting moon shadows and a much larger Great Red Spot.
The spot's "waistline" is getting smaller by just under 620 miles (1,000 km) per year while its north-south extent has changed little. In a word, the spot has downsized and become more circular in shape.

Many who've attempted to see Jupiter's signature feature have been frustrated in recent years not only because the spot's pale color makes it hard to see against adjacent cloud features, but because it's physically getting smaller.
Jupiter Red Spot_3
© NASA/ESA/A. Simon
Jupiter’s Great Red Spot or GRS is located in a ‘bay’ or hollow south of the swirly South Equatorial Belt. A titanic storm that’s raged like hurricane-like for at least 400 years, the top of the Spot’s cloud deck rises 5 miles (8 km) above the planet’s clouds and rotates in an anticlockwise direction about once every 4 days. This photo was taken by Hubble on April 21, 2014. Credit:
As to what causing the drastic downsizing, there are no firm answers yet:

"In our new observations it is apparent that very small eddies are feeding into the storm," said Simon. "We hypothesized that these may be responsible for the accelerated change by altering the internal dynamics of the Great Red Spot."

The Great Red Spot has been a trademark of the planet for at least 400 years - a giant hurricane-like storm whirling in the planet's upper cloud tops with a period of 6 days. But as it's shrunk, its period has likewise grown shorter and now clocks in at about 4 days.

The storm appears to be conserving angular momentum by spinning faster the same way an ice skater spins up when she pulls in her arms. Wind speeds are increasing too, making one wonder whether they'll ultimately shrink the spot further or bring about its rejuvenation.

Definitely worth keeping an eye on.