If you know the measurement of the object in the picture, then you just have to create an object of that size, say a cube and the adjust the image to the size of the cube. So for example I want to sculpt a playstation 5 and I have a picture of it, all I have to do is google the size of a PS5, make a cube the size with those numbers and then I scale up or down my reference image to match that size.
