How do you decide the shape and limit of polygon on power plane?
Generally you would have large areas with splits between areas of different voltages, covering the entire PCB area. Something like this: 



You would avoid having power planes where they could cause problems, such as near the mains or where they could couple undesirably to something sensitive. 

You also pull back the power plane from the edges by some reasonable amount for the accuracy of your PCB manufacturing process, say 1mm or 0.5mm so the copper is guaranteed to be buried within the PCB. More if you are using V-groove or sheared outlines. 

With most EDA programs you will be working with a negative on the power planes so you will draw tracks or keepouts to separate the areas ('split the plane') rather than to create copper polygons. 

Here is another example with 5 distinct areas defined in the power plane (the one in the centre is selected):