Hi Gabriel,
Welcome to ZBC!
You can change the Draw>Grid Size to suit the image.
For example, divide your image width or height by 1024 and then set the Draw>Grid Size to the result. This is based on a Tool>Geometry>XYZ Size for the mesh of 2 units - ZBrush’s preferred scale. ZBrush uses generic units so basically you can decide what ‘2’ represents - 2 inches, 2 meters or whatever.
This is quite a complicated way or working though. It’s not vital that you model at an exact size, provided everything is in correct proportion. You can set the scale of your model before export using the Scale Master plugin.
HTH,
Marcus