How to resize body element view size?

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CSS

CSS3

Hi, I want to know if and how I can make the body element a fixed size. For example, in this case for my webpage I gave the HTML and body elements different colors. This way, the body element is green and the HTML is blue. This makes it look like there is a green sheet on the blue background. However, if I have less text on the page, it makes the body element "smaller" as in the green only takes up a small part of the page and the rest of the page is just blue. How can I possibly make it so that the green part (body) extends to the bottom even when there is no text?

What I have tried:

Here is code so far in CSS:

html {
background-color: blue;
}

body {
background-color: green;
border: 3px solid gray;
border-radius: 20px;
}

Now here in html I only have 3 paragraphs:

Example 1........

Example 1........

Example 1........

But now on webpage, it only displays the green body background for the text. Below the text, there is nothing, so the background stays blue. How to extend the body further?

thanks in advance