SAO PAULO (Reuters) - Brazil's 2025 Arabica coffee harvest is starting with mixed sentiment as ground operations confirm forecasts of a smaller crop during a so-called off year, while bean size shows improvement, industry experts told Reuters this week.

In the Cerrado Mineiro region, where producers have collected around 4% of the expected crop, an average of 30% to 40% of beans have a screen size of 17-18, said Wellis Caixeta, coffee purchasing manager for cooperative Expocacer.

Screen size 17-18 is near the top of the scale and bigger beans more easily fill the 60-kilogram (132.3-lb) bags that coffee is sold in.

"It's very good and much better than in previous years," Caixeta said, adding the results could indicate a harvest with a better yield.

The proportion of so-called peaberry coffee beans, which resemble peas instead of sporting two flat sides, is also slightly higher, Caixeta added.

Productivity for Arabica coffee is leaning towards the lower end, confirming forecasts, said Jonas Ferraresso, a coffee agronomist who advises Brazilian farmers.

The biennial Arabica coffee cycle alternates between years of higher and lower production, with 2025 expected to be an off year.

"It's rare to find farms with high productivity this year," Ferraresso said, adding that harvesting has started in most areas.

Dry weather earlier in the season led to poorly developed and undersized beans in the top third of many trees, while coffee cherries that ripened earlier have already dropped to the ground and will likely also be of lower quality, Ferraresso said.

"I estimate that around 20% to 30% of the Arabica crop will consist of defective or lower-quality beans, while 70% to 80% will be of good quality," he said.

Arabica output is expected to decline 13.5% in the 2025 crop, said Fernando Maximiliano, coffee market intelligence manager for broker StoneX in Brazil.

While Ferraresso expects less than 10% of the crop to be harvested so far, StoneX estimates that just over 12% of Arabica has been collected as of May 26, Maximiliano said.

With low Arabica stocks in storage, StoneX expects availability to increase towards mid-June, Maximiliano said, adding that the arrival of a colder front could slow harvesting.

"Forecasts show the arrival of a polar front," Maximiliano said. "If that happens, everything will change."

(Reporting by Oliver Griffin; Editing by Richard Chang)

By Oliver Griffin