What is the requirement of the Photo

When I use HOT photo Galley Plugin. I click the generated thumbnail, the popup photo is very big, almost occupy the screen. So what is the size requirement of the photos? The photo size I loaded is 2848*4288.

Reduce their size... In example, use only 1/4 of that size.

Can you tell me what is best size requirement for the plugin?

No requirements, you can use any size. But to allow better experience for your visitors, use in example 500x750px, instead of 2848x4288px.