Print Phd thesis in A4 or A5?
Firstly, just checking the library is not enough — there might be restrictions on the format and only the relevant office can tell you what those restrictions are (if any).

Secondly, if you find out that you do have a choice (between A4 and A5 paper, for example), the natural thing to do would be to ask would be your supervisor in what format they would like to read your thesis.

(After submitting several printed copies of the thesis for review, you may get some of them back after your thesis defence, which you can submit to the library.)