MP4 conversion app - resolution size

[Nigel Ridley]

My daughter just bought an MP4 player but when I try to play video clips 
(any of the formats that the player recognizes) that I have successfully 
copied from my Feisty box, I get an error message:

'File Format Error'

Somebody told me it's because the video clip's resolution is too big.
Is there an app that will reduce the resolution size of said clips?
Is this the answer?

Blessings,

Nigel

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