An estimate from 2016 has the mass budget at 5000 kg.
Curt Niebur, outer planets program scientist at NASA Headquarters, said at the meeting that the biggest challenge of adding the lander to the Europa mission is its mass: about 8,000 kilograms, to accommodate the propellant needed to land the spacecraft softly on the surface. He added that estimate was “very rough” based on the limited studies of lander concepts to date. By comparison, the clipper spacecraft alone would have a mass of only about 5,000 kilograms.
That additional mass would drive the selection of the launch vehicle, requiring the use of the Space Launch System. “If we co-manifest the lander and clipper, we’re only on SLS,” he said. “It’s still a slow boat, so to speak: you’d still do inner solar system gravity assists.”
NASA planning for a Europa clipper alone has examined using an Atlas 5 launch and flybys to reach Europa, or an SLS launch on a direct trajectory. The SLS would be more expensive than an Atlas 5, but the direct trajectory would allow the mission to reach Europa several years sooner.
The mass limit for SLS is about 6 tons.