"折叠的大张插页" can be translated into English as "folded large-sized insert" or "folded broadsheet insert".
"Folded" indicates the action or state of being folded.
"Large-sized" or "broadsheet" describes the big dimension of the insert.
"Insert" refers to the supplementary sheet that is placed inside another publication.
Both translations are acceptable, and the choice between them may depend on the specific context or preference.