Thank you Kevin - this seems to have solved the problem
I have now updated to 2019.2 (1865), and when I select a long list of accounts, the Select Accounts dialogue does not become wide, and no longer needs to be moved to see the buttons. So far, success!
I note that the long list of accounts doesn't wrap onto new lines, just gets chopped off at the right. A very minor problem, and easily worked round.
I have also learnt from you about the preview builds and change log - thank you. I had assumed from the Download Page that 2019.1 was the latest version.