A couple of added considerations from me;
I used to take my own OS map (to work) and print off the section(s) I intended using for walks, thereby saving my map from weather damage and prolonging it's life. I would fold the A4s in half and insert them into one of those clear plastic wallets from stationers.
I found the half size = A5 was easier to stash in my jacket chest pocket.
A while later I realised you could buy an Ortleib map case of A5 size. Despite potential navigational shortcomings of doing this, I never found it a problem.
The only downside is, that over the years the clear Ortleib has discoloured and instead of being a clear plastic, I now have maps with a distinct sepia tinge!
I should hold my hand up and admit that my last map case was bought in about 2008/9 - so perhaps they are expected to discolour after so many years
It is probably time for me to buy another new one, although I do have the standard big size case which was shelved when I moved onto the smaller version. Perhaps I'll give that an airing.