。
She was wearing a long, loose coat.
Literal
She [topic-は] long [and-くて] relaxed coat [object-を] was-wearing.
長くてゆったりとした chains two adjectives: 長い ('long') via its て-form, and ゆったりとした ('loose/relaxed'). ゆったり is a 擬態語 describing something spacious and unhurried — in clothing, it means loose-fitting. ~としたCoat modifies コート as a prenominal adjective. 着ていた captures the ongoing state of wearing.