I had this yesterday mate...
I'm using an Epson printer, in the print settings box thing, I found a menu called "Paper Handling".
In said menu there is an option for "destination paper size".
For some reason, the destination paper size was not the same as what I told it I was putting in (I was doing a 5x5 print).
Once I altered that to match, the print came out exactly the size I wanted.

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