There is an 18-gallon fuel tank limit in F1 for the entire race. The rules allow more fuel burn so they decided to cut ten miles from the race.
No fueling at pit stops?
Fuel tank size isn't specified in the TR. 18 gallons would only get them about 2/3rds the distance of a 305 km race. The 2026 cars are actually less fuel efficient than the 2025 cars were because they deleted the MGUH (which reclaimed heat ordinarily wasted by the turbocharger).
The reason they're shortening the races is the packaging of the 2026 cars was based on a 305 km race with fuel flow limited to 3000 megajoules in an hour.* The packaging is very tight and the fuel tank size can't be increased without triggering another round of homologation.
In answer to complaints (from both fans and teams) that the cars are running out of battery power too frequently, for 2027 they're increasing the internal combustion engine's maximum fuel flow to 3150 mj/h. These cars are turbocharged and they're nowhere near maximum output, even with the increased fuel flow, and extra ICE power from the extra fuel somewhat offsets the car's inability to harvest enough energy to keep the batteries energized. But it also (obviously) reduces fuel economy, and the fuel tanks that were designed for a 305 km race are no longer big enough.
So because the FIA failed to anticipate the possibility that it might be necessary to increase fuel flow, they're having to shorten next year's races (all except Monaco, which is only 260 km to start with).
*They're now measuring the energy content of each engine supplier's man-made synthetic fuel, and the flow rate is based on that energy content rather than its volume or weight. Which makes it exceedingly difficult for fans to understand because each of the six engine suppliers makes their own (synthetic) fuel and they all will have a different weight or volume based on their energy content. The fuel economy measurements in common use -- miles per gallon or liters per 100 km -- are no longer relevant.