First off, make sure you’re building your lighting on production. Area lights won’t resolve well on lower quality bakes. If that fails you may have to raise your ray count.
When you have a light with a very large source radius it will require a lot of shadow rays to resolve. By default Lightmass uses 16 penumbra shadow rays on Production Quality for area lights, which is good enough for most realistic source sizes, but not for larger ones. For that you’ll have to bump the ray count up, sometimes by a very large amount.
You can increase the penumbra rays by editing the setting in the Lightmass ini file in your engine (or more ideally, project) directory. This will probably affect bake times and memory consumption during the bake, but I don’t know how big the impact will be.
