I have a copy of the Kodak Reference Handbook from 1944, and it describes the Kodachrome process and prints available. There were two types, Kodak Minicolor for prints from 35mm slides, and Kotavachrome prints from the professional fims, which were sheet Kodachrome - available in sizes up to 11x14.

Quoted from the book: "Instead of a paper base, a white, pigmented cellulose acetate support is used for these prints. This gives them the feel of a fine playing card."

And:

"Kotavachrome Professional Prints are primarily intended for the commercial field. Each order receives individual attention, and the customer's instructions regarding the making of the print are followed whenever possible. Such instructions should be explicit to avoid misunderstandings. Every effort is made to assure the finest possible reproduction of the Kodachrome originals."

It goes on to further state that the max size is 11x14, and standard sizes are 8x10 and 11x14. Prints are made from all sizes of original except 45x107mm, 6x13 cm, and 11x14 inch, so that says that it isn't a contact printing paper. Cropping can be specified, so it sounds like it was a custom lab service.

In the film section it lists the worldwide processing locations and the only two that did the sheet film are Rochester and Hollywood.