Semantic UI does not require anything out of the ordinary, to increase the font size simply by using one CSS class:
.cabecalho{
font-size: 250%;
}
<span class="cabecalho"></span>
If you'd like to check out some pre-defined sizes by Semantic UI itself, take a look here / a>.
Here is an example of a dropdown I found on the site itself:
<div class="ui selection dropdown">
<input type="hidden" name="gender">
<i class="dropdown icon"></i>
<div class="default text">Gender</div>
<div class="menu">
<div class="item" data-value="1">Male</div>
<div class="item" data-value="0">Female</div>
</div>
</div>
To change the font size of items within the drop, simply change the css class, just be careful that this code can change in other places that use the class .item
also, any questions leave a comment here in my answer .
.item{
font-size:22px;
}