Your suggested size is not an issue.
I would say that this albatross is roughly the size you're after?
The existence of the albatross in real life proves that the size of the animal in no way precludes it from flying.
Not convinced? How about this bad boy?
That's the Argentavis Magnificens, and it is massive.
The Andean Condor was mentioned in another answer and is an equally relevant addition to the list of massive birds.
I don't think they are necessary. There are a few interesting things to note about hollow bones, and I suspect these differ from your current interpretation of why hollow bones are useful:
With hollow bones a bird can fly very long distances without getting worn out from carrying its own weight.
The benefit from hollow bones isn't so much that they allow you to fly, but rather than they minimize exhaustion. Once you're flying, your torso is essentially suspended from your wings. The heavier your body is, the more stress that puts on your shoulders.
Unless your flying mammal is capable of long-distance flight, the hollow bones don't seem all that necessary. But there's nothing wrong with giving the animal hollow bones.
You might think these bones are fragile, like empty egg shells, but birds can't afford to have bones that break easily. The hollow bones are supported by internal struts – structures inside that help brace the bone so that it can withstand longitudinal pressure (pressure along its length).
As per your question:
But could hollow bones support the weight of such a animal?
Given the explanation quoted above, I'd say that the hollowness of the bones does not meaningfully affect the animal's capability of supporting its own weight.
Birds have hollow bones, and let's look at avian weight record holders:
- The largest bird in the fossil record may be the extinct elephant birds (Aepyornis) of Madagascar, which were related to the ostrich. They exceeded 3 m (9.8 ft) in height and 500 kg (1,100 lb).
- The largest carnivorous bird was Brontornis, an extinct flightless bird from South America which reached a weight of 350 to 400 kg (770 to 880 lb) and a height of about 2.8 m (9 ft 2 in).
Those are massive birds, which proves that it's possible.
Do note that there are no currently living flying birds that weigh more than e.g. a labrador. It seems that heavy flying creatures are at a disadvantage (unless they went extinct for an unrelated reason?).
But that doesn't mean that yours can't exist. You're talking about a completely different ecosystem where most of the wildlife flies. It's perfectly possible for a heavy flying animal to not go extinct.