Please anyone clarify me what should be the max-min length of the iPad/iPhone application can be?
The length that it's technically allowed to be is a lot longer than you should make it. The reason being iOS will abbreviate the name on the home screen if it won't fit in the space available, for example "Really Long Title" might display as "Reall...le".
I say 'might' as iOS does not use fixed-width text. The only way to see what works is by trying it and seeing what works.
Finally, the display name is not necessarily the same as the application's name. For example, both the full and free versions of my app use the same display name, though they have different names in the App Store.