Far too often self-published authors print their books in the wrong format/size.
Before you begin your conversation with a printer go into a bookshop and do some research – look at the other books that have been published in the same genre that you have written, and make notes of the dimensions / finish / binding of the books on offer (know that A4 and A5 are never/seldom used, and this is what a printer will quote you on if you do not give them proper specifications).
Here are the most used, publishing industry, recognised formats:
Hardback – 235mm high x 153mm wide (serious books such as histories, biographies, politics, finance) and usually has a dust jacket
C Format (aka Trade Paperback) – 235mm high x 153mm wide (same size as a hardback, but bound like a paperback)
Demy – 209mm high x 137mm wide
B Format – 198mm high x 130mm wide (paperback)
A Format – 181mm high x 111mm wide (mass-market paperback, mostly fiction)
And for genres such as coffee-table books, gift books, cook books, children’s board books and picture books, etc. there are various formats that you could use – so once again see what is out there on the bookshop shelves before making a decision.