how i can find web page total height (vertical length)

Answer 1

Use 100vh(viewport height).
Similar to 100% but it shows the 100% of the height of the viewport (i.e, the screen) and not of the parent element.
Eg-
div{
height:100vh;
}

Answer 2

Use guess and check, eventually you will get a matching height.
Or be lazy and just use 100%

Answer 3

If you want to find viewport height in pixel
viewportHeight = window.innerHeight;
https://code.sololearn.com/W4Mbn5gSd2fc/?ref=app