Default Modify system.img size

Hi, I'm modifying a rom 4.2.2 allwinner a20 tablet. and would like to change my system.fex for backup system.img but have different file sizes. I need help to see where I need to change the size information. Because when you want to load by phoenix suite tells me an error.
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Name:	rom.jpg
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Size:	131.8 KB
ID:	2575657