You can also use a unit-less value, which is the number of lines: line-height: 2; is double spaced, line-height: 1.5; is one and a half, etc.

You cannot set inter-paragraph spacing in CSS using line-height, the spacing between <p> blocks. That instead sets the intra-paragraph line spacing, the space between lines within a <p> block. That is, line-height is the typographer's inter-line leading within the paragraph is controlled by line-height.

I presently do not know of any method in CSS to produce (for example) a 0.15em inter-<p> spacing, whether using em or rem variants on any font property. I suspect it can be done with more complex floats or offsets. A pity this is necessary in CSS.

If you want condensed lines, you can set same value for font-size and line-height

In your CSS file

.condensedlines {              
    font-size:   10pt;
    line-height: 10pt;  /* try also a bit smaller line-height */
}

In your HTML file

<p class="condensedlines">
bla bla bla bla bla bla <br>
bla bla bla bla bla bla <br>
bla bla bla bla bla bla <br>
</p>

--> Play with this snippet on jsfiddle.net

You can also increase line-height for fine line spacing control:

.mylinespacing {
    font-size:   10pt;
    line-height: 14pt; /* 14 = 10 + 2 above + 2 below */
}

Try this property

line-height:200%;

or

line-height:17px;

use the increase & decrease the volume

I am not sure if this is what you meant:

line-height: size;

Try line-height property; there are many ways to assign line height

Yup, as everyone's saying, line-height is the thing. Any font you are using, a mid-height character (such as a or ■, not going through the upper or lower) should go with the same height-length at line-height: 0.6 to 0.65.

<div style="line-height: 0.65; font-family: 'Fira Code', monospace, sans-serif">
aaaaa<br>
aaaaa<br>
aaaaa<br>
aaaaa<br>
aaaaa
</div>
<br>
<br>

<div style="line-height: 0.6; font-family: 'Fira Code', monospace, sans-serif">
■■■■■■■■■■<br>
■■■■■■■■■■<br>
■■■■■■■■■■<br>
■■■■■■■■■■<br>
■■■■■■■■■■<br>
■■■■■■■■■■<br>
■■■■■■■■■■<br>
■■■■■■■■■■<br>
■■■■■■■■■■<br>
■■■■■■■■■■
</div>
<br>
<br>
<strong>BUT</strong>
<br>
<br>
<div style="line-height: 0.65; font-family: 'Fira Code', monospace, sans-serif">
ddd<br>
ƒƒƒ<br>
ggg
</div>

lineSpacing is used in React Native (or native mobile apps).

For web you can use letterSpacing (or letter-spacing)