border-width
Syntax:

border-width: thin | medium | thick
border-width: absolute units designator (cm, mm, in, pt, or pc) or a relative units designator (em, ex, or px)


<style type="text/css">
.demo1 {
   border-width: thin;
   border-color: #FF0000;
   border-style: solid;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo1"></div>
<style type="text/css">
.demo2 {
   border-width: medium;
   border-color: #FF0000;
   border-style: solid;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo2"></div>
<style type="text/css">
.demo3 {
   border-width: thick;
   border-color: #FF0000;
   border-style: solid;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo3"></div>
<style type="text/css">
.demo4 {
   border-width: 10px;
   border-color: #FF0000;
   border-style: solid;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo4"></div>