Letters ~ Large
To qualify as a Royal Mail Large Letter the maximum size is limited to 25mm x 250mm x 353mm.
To qualify as a Royal Mail Large Letter the maximum weight is limited to 750gm.
The weight boundaries for Large Letters are 100gm, 250gm, 500gm and 750gm (the maxiumum).
An item over these limits is likely to be a Small Parcel.
25mm, one inch, is about the thickness of a 400 page paperback book. The size of a C4 envelope is comfortably within the boundary, and such an envelope can usually be filled quite full with sheets of normal office paper before the thickness limit is reached. A sheet of A4 paper weighs around five grams, and a C4 envelope about 15gm ~ so 16 sheets of 80gm paper is about the limit for the lowest weight boundary.
In Imperial terms, 1lb is 454gm. A B4 sheet of paper is within the limit, but if it is wrapped in any way it will be out of range since there is only no spare to allow for the wrapping's overlap, let alone for any form of protective stiffener.