Depends on your mesh… I would never go higher than 2048 (even 1024). I would break up your floor per room!
And yes this can be fixed at the basics: in your 3d app… And no “making objects floating” won’t help!!: whenever you want to get proper shadows at corners/edges you’ll have to edge snap walls/floors to each other OR break up your mesh and keep the lightmap uv islands only for the visible, non overlapping parts!