Size of models does not impact texture quality or mesh. Visual quality will remain regardless the scale of your objects. Well, at least to some degree because your are limited by floating point precision. It’s unlikely but If you make your models extremely small or extremely big they will suffer from precision errors during rendering. What you should be aware of is the is the limitation of engine’s world size and object’s position in world space(world space position is a floating point vector). I’m not up to date with Unreal’s newest open world system but it appears to be quite powerful. It’s up to you to judge scale of your models. As guys mentioned, keep your models scale relatively consistent to each other and try to fit whole universe within UE world boundraries.
Btw, If you think about using VR, things get complicated because you will have to override interpupillary distance(IPD) and scale positional tracking vector accordingly to your universe scale.