The thrift paper size is 280x380.
I use it when posting B4 papers. B4 is 250x353. I sandwich the B4 sheets centrally between two boards of thrift size. This provides a margin all round, sufficient to allow for some crushing of the edges and corners in transit.
Although the margin all round is small, it does make the overall package size jut too large to meet the Royal Mail 'Large Letter' sizing. The maximum Lare Letter size is exactly B4. So a pile of sheets of B4 could class as a large letter if they were posted unwrapped. Even a tight wrap of brown paper would make the bundle too thick to pass the RoyalMail test. You might get away with that, but the recipient would not be pleased at having to deal with 'incorrect postage' delays and costs. Also passing papers of any value through the post will necessitate some form of protection against creasing.
So why bother to explain all this? When I have a pile of used cardboard boxes, and a spare half-hour I cut some 'thrift' boards. I need to remind myself of the sizes I use. This is just one. It occurs to me that selling colleagues might work in the same way, and there is nothing secret of patented about it. I can remember thrift - but not 280380.
Other floral aide-memoires used by me are listed at [112015].