In CSS, you can adjust the height and width of elements using the `height` and `width` properties.
For instance:
```
div {
height: 100px;
width: 50%;
}
```
In the above CSS rule, the height of `div` elements would be 100 pixels and the width would be 50% of the parent element.
You can use different measurements for these properties such as:
- Pixels (`px`) for absolute measurements.
- Percent (`%`) for proportional measurements relative to the parent element.
- Viewport height (`vh`) or viewport width (`vw`) for proportional measurements relative to the viewport or screen size.
- em or rem for measurements relative to the font size.
An Example of `vh` and `vw`:
```
div {
height: 50vh;
width: 50vw;
}
```
In the above CSS rule, the `div` will cover 50% of the viewport’s height and width regardless of the size of the viewport.
An Example of `em`:
```
div {
height: 2em;
width: 3em;
}
```
In this CSS rule, the `div` height and width will depend on the size of the element’s font. One `em` is equal to the current font size. If you set the font size to 16px, for instance, then the `div` would be 32px tall and 48px wide.
Note: If you want to include padding, border and margin into the element’s size, you can use the `box-sizing` property and set its value to `border-box`.
For example:
```
div {
box-sizing: border-box;
height: 200px;
width: 200px;
}
```
