The phrase "decline in size" is correct and usable in written English.
You can use it when referring to a decrease or reduction in size. For example, "The population of the town has seen a decline in size over the past few years."
Concurrently, the fin-ray section of the fin would decline in size.
Starting in 2015, this group is expected to noticeably decline in size.
One day, when the portions began to decline in size, Oliver asked, "Did the price of beef go up?
During the 6th dynasty, which Manetho also designates as Memphite, the funerary monuments in the pyramid field of Ṣaqqārah continued to decline in size and workmanship.
The growth of agricultural production in the 20th century, accompanied by a decline in size of the rural population, however, has given impetus to research in agricultural policy.
The prediction for Saturday was for the wind to drop to about 7 mph and the waves to decline in size to about eight feet, but the forecast for next week turns sour again.
Peak adolescent fracture incidence at the distal end of the radius coincides with a decline in size-corrected bone mineral density (BMD) in both boys and girls.
Our yuletide chocolate treats are reportedly increasing in price while declining in size.
Heathland areas in the Netherlands are declining in size and quality, and becoming more fragmented.
Newspaper newsrooms declined in size by forty-five per cent between 2008 and 2017, according to a study by the Pew Research Center.
Hardwood patches declined in size, number, total area, and within-patch cover-type heterogeneity, and became more irregular in shape.
