Can you help me resize a video for the iPhone?

I'm having a hard time getting a video resized to where it will show correctly on the iPhone.  I have an iPhone 5 running OS 6.1.4 and I've tried resizing my presentation to the following resolutions:

640 X 480

1280 X 720

1136 X 640

In every case the vido is too large.  In the case of 640 X 480 it doesn't fit and it loses too much resolution.

If you've had luck resizing presentations to work on the iPhone, then please share the secret of your success.

Thank you!

Brad