Ideally, the application compiler will use the same number of pixels that are present in the splash screen that you specify in the GUI while building the application.
However if this does not work for you, as a work around for this problem you could do the following
1) Navigate to the folder where the exe is present.
2)Replace the 'splash.png' with the required file that you want to use
3)rename the file as splash.png
If this does not work,
Try a smaller picture, lets say a 200x200 png file.