Paper sizes follow an international standard which we are all aware of:
A4 210mm x 297mm
A5 148mm x 210mm
A6 105mm x 148 mm
When is comes to note books the size may be as per the standard paper size but often they are not, and you need to check carefully. Often a notebook described as A5 can be marginally bigger or smaller.
Moleskine tried to set a standard (which they use) but it is not universal:
PK 90 x 140mm
XS 60 x 105mm
L 130 x 210mm
XL 190 x 250mm
There are variants for example one manufacturer has the PK or Pocket size as 93 x144mm and the large is a genuine A5. All of this may not matter to you – but do be aware.