Roughly A5 size.

As you can see pages 1 & 2 have largely disintegrated (although I seem to have most of the pieces if you like jigsaws!) The bottom right page corners are also very fragile, but there is no loss of to the remainder of the booklet apart from pages 1 & 2, with the other pages in generally good order.  The final page of spare parts listing is also fragile but intact.

Estimate 1930's as I believe the OHV models were introduced in 1935.