The earliest record concerning a flat desk is made in 1713 in the inventory of a merchant, showing that the come back of luxury in France did not wait for the Régence.
André-Charles Boulle began his career half a century earlier and is the most notable furniture designer of his time. At the time of the Régence, he acts as an entrepreneur, seconded by his four sons and employing many workers. His workshops are in the Louvre.
Boulle standardizes his production of bureaux plats in two sizes, 5 pieds and 6 pieds long. Some exceptional pieces are larger, such as the desk of 6 pieds 8 pouces sold for £ 2.9 million including premium by Christie's on December 14, 2005.
Boulle workshops at the Louvre burned in 1720, possibly by arson. The inventory provides interesting information about the activity. The extraordinary art collection assembled by Boulle was also destroyed.
The waiting pieces were quite numerous, suggesting that the elements were prepared in advance before final assembly and decoration to customer requirements, enabling to reduce the time between order and delivery. It also appears that the most common model was the 6 pieds with a top of 6 x 3 pieds, 195 x 98 cm.
Restarted after the fire, the production of these models by Boulle ceased around 1725.
On September 18 in Zurich, Koller sells a bureau plat of six pieds attributed to Boulle, richly decorated in première-partie (dark background) and embellished with false drawers in the back. The faces are centered with masks, Satyr in the front and Democritus in the rear. It is estimated CHF 1.5M, lot 1078 shared on Invaluable.
Note that this standardization of dimensions was not limited to Boulle. A bureau Régence attributed to Gérard discussed in this column in 2011 had the same size for the top and also for its height at 2 pieds 6 pouces.
I used the old French units, but they were little different from the English values, foot and inch.