Squarespace creates up to seven versions of that image, should I have the same?

I read here

When you upload an image, Squarespace creates up to seven versions of that image, each with a different width. These widths are:

* 100 pixels
* 300 pixels
* 500 pixels
* 750 pixels
* 1000 pixels
* 1500 pixels
* 2500 pixels

I have seen several reports like the one above for image sizes.

I am using a background slider for this issue.

Your slider picks the large image.

So
1. Are the 7 sizes above something that I should do ?
2. Would you recommend a plugin that you have used to make the 7 sizes or can wordpress be set to make them?
3. How do I set the 1500px image to LARGE ? I assume that is what would be the appropriate image to use given what you wrote above.