Hi Afshin,
I observed that you are using a resolution of 600 DPI which is why you are getting one block saved in the image. Reducing the resolution size will save the entire block diagram in the image.
Refer to the following documenation link which explains you to choose the right resolution for your use case: https://www.mathworks.com/help/matlab/ref/print.html#bukyb6e-1-resolution
To print a very high resolution image, you could use the SVG (Scalable Vector Graphic) format as that scales well.
The following command shows how to use the svg format.
>> saveas(get_param('untitled','Handle'),'MySimulinkDiagram.svg') % where 'untitled' is the name of the model
You could then convert the svg file to png using an online third party tool. See the following link to convert the file. There are more tools available online.https://svgtopng.com/
The other possible high resolution image can be obtained by printing a model to PDF format.
I hope the above information helps.
