Also: do realise that your screen probably has a higher pixel density than the document is set up for. The document is most likely set to 72px per inch (that's the default), while the screen is likely at 96 or a bit above 100, making everything seem smaller (because Photoshop reading the document thinks that 1inch is 72px and so says "100%" at that view size, while perhaps 1inch should be much higher.

Example: My screen (a 22" 16:10 screen) is 473mm and 1680px wide, meaning it has a pixel density of about 88px/inch, and Photoshop still suggests 72px/in as the resolution for every new image I make.

I would probably recommend you work at about 300px/inch as the printer has a much higher resolution than your screen. But regardless of the resolution of the image, Photoshop will talk to the printer in mm, so if you say the image is 140x140mm, that's what the print will be regardless of how many or few pixels you use.

And yeah, I realise I'm switching between mm and inches here, because pixel densities are nearly always set as px/inch rather than px/cm, but I think in mm. 1in is roughly 25.4mm. That was case covers, and ShedoSurasu mentioned having seen it.