What do for font size in html

Question

Because you not tell me to how to do

Answer 1

< p style="font-size:4px">html</p>

Answer 2

I see you also starting CSS, so I better show you how to do it in CSS.
Suppose your HTML has this:
<div class="formatted" > this line is formatted </div>
<div class="notformatted> this line is not formatted </div>
In CSS, you need to
.content {
font-size : (your font size) ;
}
Please setup a blank web code with above first. Then you can test the following.
Here you have four choices.
The first type is only number. For example, you can input 1.5 and compare with no 1.5 and see what the difference is. Then you can try 2, and then 0.5
No unit is scaling. The number will be ratio
Second type is fixed, you add a unit. You can begin with 4px as Charancr suggested. You can try 10px, 20px, 30px. After that, 1em, 2em, 5em.
Third type is 30%, 50%
Fourth type is 5vh, 20vh, 10vmin.