If the image on a real tank was 10 cm in size, then in scale it should be:
1/56 = 1.79 mm
1/48 = 2.09 mm
1/72 = 1.34 mm
Which option is closer to what you want?
You can use decals for a different model than the one intended by the manufacturer. For example, you can look at decals for a truck, motorcycle, etc. They come in a variety of sizes and might be perfect for your model.