Each icon is 32x32 pixels (unless you've changed the icon-size).
If your view is 6x6, then you've got 6 icons by 6 icons that will be displayed on the map at a time. For the width of the title screen you want to make, you multiply 6 icons by 32 pixels which is 192. For the height, you multiply 6 icons by 32 pixels again.
Therefore, your picture needs to be 192x192 pixels...
If your maps icon-size is not 32, then you should multiply 6 by whatever your icon-size is.
6 x 32.