The debate over what size Liz is, exactly, need not continue. She is not XS to S, so she wouldn't fit the skirt and the bodice wouldn't fasten in the waist area. Her chest might not give trouble, but her waist would.
For those who are confuzzled about why this is a Big Deal, the dress under the house is NOT the same dress Marian was drawn wearing in the past. The dress the Costume Nazi is referring to is from a strip where Jim is thoughtbubbling him and Marian at their wedding and she is clearly wearing a Victorian era 'dress'.
The preservation issues facing a dress kept in a crawlspace are a whole other discussion, regardless of style. Someone suggested perhaps the Pattersons chemically treated the gown to get it white. All who responded to that agree that would seriously damage the gown. Since that action defies all logic we here possess and would render the gown unwearable, it strikes the Costume Nazi that this is exactly what the Pattersons did since Foobverse logic seems to run counter to the way it works in reality. ;)