how to comapare big screenshots

If you compare against a screen, the image must be fully visible.

Another option is to compare images using class Finder.
In case you can use a screenshot tool, that makes a shot of a complete website including what is not visible.

An Image which needs to comapare with screen is fully visible inside the screen.

My que: my Image is big in size whether it will find or not. even the image have breakage in bottom-Right of that image.

--1. if the shot image has width x pixels and height y pixels, then all x * y pixels must be visible on the screen.

--2. how many pixels in the shot and on the screen are equal makes up the similarity score: the more pixels are different the lower the similarity.

If a few pixels differ you will get FindFailed with wanted similarity 0.99.

Generally: large images tend to make problems, since the more details, the more the risk, that they have changed somehow on the current screen.

It might be better to work with 2 or 3 smaller detail shots, to verify a larger image.

Thank you