Shorthand to set height and width of an element in CSS

you can use css variable

/* css file */
:root {
  --length: 50px;
  --ratio: 1;
}
.box {
  background: cornflowerblue;
  width: calc(var(--ratio) * var(--length));
  height: var(--length);
}

<!-- html file -->
<div class="box"></div>

then you can change --length in many ways and box width and height will respect to changes. and Its better method than the SCSS variable for debugging purposes.

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There is no short hand for setting the height and width of the element in a single property declaration. You cannot do it with SASS as well.

But yea, SASS will provide you a feature to hold the common value shared in both property by declaring a variable like

$some-var-name: 100px;

.some-class {
  height: $some-var-name;
  width: $some-var-name;
}

As I said, even SASS won't help you writing height and width at the same time but you can use change the value of both from a single variable.

---

Ok I was about to add the @extend in the answer but since other user has already answered the same, (which is now deleted)

.size{
    height:100%;
    width:100%;
}

element {
    @extend .size;  //Sets element to height:100%;width:100%;
    // more stuff here
}

I would suggest you to use a declaration of % instead of . so instead of using .size suggested use %size. This way your literal class of .size used only for extend purpose won't be included in the compiled stylesheet.