Basically creating splash screens for Android applications is a very bad practise unless it's a game.
Your questions sound a bit unclear, maybe you could be more specific. However, I will try to help you.
First of all you should get familiar with this Supporting Multiple Screens. It's not a link to SO site.
On that web page you will find the following picture.

When you design for Android think about sizes in DPI. Your reference size bucket would be MDPI. If you create a bitmap 100x100px then you have to create separate bitmaps for other densities using factors: HDPI 1.5 (150x150px), XHDPI 2.0 (200x200px) and so on.
Splash screen is something that usually covers full screen but it can be created from parts. For example create NinePatch bitmap that is automatically scaled for any screen size, usually buttons use NinePatch for backgrounds. Next create separate bitmap with a logo or any other graphic that you wish to present. Avoid whitespaces in your bitmaps.
Parts of the splash screen would be joined and displayed by developer so you should not care about merging them anyhow.
I don't know if that's what you expected. I will try to update my answer if you provide more input.