The image is made of pixels (obviously always an integer number of them), and the print size is obtained by multiplying the size in pixels by the print definition in pixels-per-inch or pixels-per-millimetre. In other words there is no true physical size.
Now it all depends of the size in pixels (which is something you don't mention). You can set the print size to anything you want using
Image>Print size and changing the print resolution. The image will appear pixelated if the print definition is below 100PPI. How acceptable this is depends on the image content. To give you an idea of print definition:
- Last consumer-grade CRT displays: 72-80 PPI
- Current LCD: 100-120PPI
- High definition LCD: 200 PPI
- Draft paper printing: 150 PPI
- Standard office printing: 300PPI
- High quality office printing: 600PPI
Note that your scanned image has not got the right aspect ratio for an A4 (1238.25/899.58 is 1.38 when it should be 1.41) so you may have to crop it a bit.