From the booklet An Account of the making of the Oxford Lectern Bible by Bruce Rogers,[1] "...for the size of the leaf, I now learned for the first time, must not exceed the standard dimensions of the folio Bibles used on the brass lecterns of most English churches — and this size was 12 x 16 inches.
So I proposed to Mr. Milford that he have fifty copies printed for me on larger paper, preferably a hand-made, to which he readily agreed; and eventually two hundred copies were printed on Batchelor's paper in this form, in addition to the printing of 1000 copies on the smaller paper made at the Wolvercote Mill."
A volume of this size being inconvient to handle except on a lectern, 200 copies intended for general distribution were divided into two volumes, duplicate title pages and suitable half-titles being supplied.
The third size was the 19-1/4 by 14 inches for the unique copy in the Library of Congress according to the Arion Press.
Several images can be seen on the page for The Oxford Lectern Bible at White Unicorn Books.