This isn’t anything to do with printer settings, but with the size of the image as you indicate it to the program you use. So yes, you need to know how big that engine is in real life, then measure up the image you have, and do some maths to work out what resolution you need to print it out at.
Two things to remember here. One is that you’ll want to use the largest dimension you can find (and measure in the photo), because this reduces the margin of error. That is, if research turns up the height of the whole engine and the diameter of the flywheel, using the height of the engine is better because it’s bigger. The other is that your photo has perspective, which means it’ll be hard to make good measurements in it from left to right, but the engine is pretty much straight up, so vertical measurements will be fairly accurate.
Here’s an example that will hopefully show you how to do this. Say I want to print out this drawing in 1/35 scale:
It helpfully includes a scale bar along the bottom, which photos of course tend not to have (unless they’re American official photos of tanks under test).
If I open this in Photoshop (or pretty much any other image editing software), it’s trivial to measure the length of the scale bar using the measuring tool (in Photoshop to be found under the darker button you can see on the left in this image, though this normally shows an eyedropper — you'll need to keep your mouse button pressed on that button to pop out a menu so you can select the ruler tool):
This together lets us do maths. The scale bar represents 20 feet, which is 609.6 cm in real-world measurements. Image resolutions are typically expressed in dots (or points) per inch (I blame the Americans) or per centimetre; in the drawing, the bar’s length is 361 pixels, so divide 361 by 609.6 to calculate that the drawing’s resolution is 0.5921 pixels per centimetre for the real tank. That needs to be multiplied by 35 to get it to 1/35 scale, giving 20.72 pixels per centimetre.
In Photoshop, you can change the image’s resolution in this window, reached via the menu Image → Image size:
We don’t want to resize the image but only change its resolution, so a quirk of Photoshop to be aware of is that in that case, you need to remember the width shown in the first text box, in this case 1014. The reason is because when changing the value in the Resolution text box at the bottom, see what happens:
Yes, it changes the pixel size of the image so that its size in centimetres stays the same (note I also changed it from pixels per inch to pixels per centimetre). So all you do is type the original value there again:
(Note it’s easier and less error-prone to do this by copying the value — select it, then press ⌘C on a Mac or Ctrl+C on Windows — then paste it back with ⌘V or Ctrl+V after changing the resolution.)
Then just hit the OK button or press the Return key and the image is resized to the correct dimensions.
This doesn’t require Photoshop: other image editing programs, including ones available for free, can do this too — their windows etc. may look slightly different, but the technique will be exactly the same.