20% +/- Square footage "Rule" for Comparing Comps

[NA Zeld]

Can anyone point me to any written rule or guideline that specifies the max percentage square foot differential between two properties for the purposes of calculating an appraisal?   I've seen lots of posts stating that comps should be within 20%, but it would be very helpful to me if I could find some authority for that number. I've run into a situation on a subject property where the comps used are at 30 to 45% of the size and I'm looking to contest. For what it is worth, there are other properties closer to the subject in size, but those sales are more than a year old.

Any help GREATLY appreciated. Thanks!