The cover is the thing you are paying to brand, and it should still be in use long after the paper inside it has been replaced many times over.
Stock sizes only
Letter for full-size covers, 5 x 8 for junior ones. Both are sold by every stationer and will still be sold in five years.
A cover cut around a bespoke pad looks identical on the quote and becomes landfill the day the original refill runs out, because nobody will re-order a special pad for an old folio.
How the pad is held matters as much as its size
A board retainer or an elastic lets a pad be swapped in seconds. Glue does not.
A glued pad also sheds: the adhesive strip lets go in a warm bag and the pad ends up loose at the bottom of it, which is the second-most-common complaint we hear.
Ring binders are the same argument
The reason to buy a ring folio is that pages come out and go back in. That only holds if the mechanism is riveted through the back board.
Glued and heat-welded mechanisms peel under the weight of a full manual and usually take a strip of the lining with them.
What this means when you order
Ask for the pad size by name, ask how it is retained, and ask what a replacement pad costs at a normal stationer.
If a supplier cannot answer all three, the cover is being sold as though it were disposable, whatever it costs.