According to The Guardian, the size of the pumice raft is around 58 sq-miles and it was the result of an underwater volcano.

newly formed pumice raft between Tonga and Fiji in the Southwest Pacific.

It erupted about August 7 and was captured by satellite images. This 195 km2 raft extended to the horizon and consisted of a 15–30 cm thick layer of freshly erupted pumice clasts, implying a minimum estimatedvolumeof30millionm3 (Figure1).Thepumiceraftwas2daysold,anderuptedfromasubmarine caldera volcano at ca. Drifting chiefly westward, the raft reached the Fiji Islands on the 19 September. "Each piece of pumice is a rafting vehicle. Pumice is a porous volcanic rock. She wrote: “We sailed through a pumice field for six to eight hours, much of the time there was no visible water. A HUGE floating mass of pumice rocks has been discovered by sailors as they made their way to Fiji. An Australian couple saw it first on August 17 as they were sailing a catamaran to Fiji. Yachts that crossed the raft as early as 2 days post‐eruption provided an outstanding data set of raft characteristics and pristine samples. The pumice raft holds promises. Michael Hoult with some of the pumice he encountered sailing near Tonga. “It was like ploughing through a field. QUT geologist Associate Professor Scott Bryan will be studying samples of the pumice recovered by passing sailors shortly after the eruption. On the 7 August 2019, a 195 km 2 raft of andesitic pumice was produced at 200 m below sea level at an unnamed submarine volcano in the Tonga Islands (Southwest Pacific Ocean).

200 m below … Sailor Shannon Lenz shared footage of the phenomenon online. The pumice raft was first spotted in the waters of Tonga in August. ‘Osi mei ai ha uike ne a’u e lahi e mafola ‘a e maka tēkina’ ni ‘o liunga ua he lahi ‘o e kolo ko Manhatan ‘i Niu’ioke. A giant pumice raft has been spotted in the Pacific Ocean, near Vava’u, Tonga. Pumice raft at sea after underwater eruption near Tonga ‘Asi ha fu’u tu’oni maka fōfō’anga ofi ‘i Tonga hili ‘eni ha mapuna ‘a ha mo’ungaafi he konga ki mu’a ‘o e māhina ni’. He says by that time, the raft will be "covered in a whole range of organisms of algae and barnacles and corals and crabs and snails and worms."