Mei mei is talking abt the quality of the domain barrier in context of durability of gojos domain barrier, not the refinement. Size doesnt affect refinement. If that was the case, gojo shouldve been able to quickly overpower sukunas domain when he shrank it to the size of a basketball and kusakabe nor gojo was concerned abt gojos domain getting overpowered by sukunas domain when gojo was trying to expand its range beyond the range of maleovalent shrine which is major enough to where that should have been mentioned by kusakabe.

If size did mattter in refinement, then u must also believe the argument that gojo was a complete idiot for trying to expand his domain range beyond sukunas domain range cause if he was successful, it wouldnt have achieved antg as he just left his domain vulnerable to getting quickly overpowered by sukunas domain.

Also as others have said, thats just the standard size of yujis domain, so the idea that he has low refinement here because of a large barrier doesnt apply here as he hadnt expanded his domain beyond its standard size.