From http://www.w3schools.com (Copyright Refsnes Data)

CSS border-width Property

Example

Set the width of the four borders:

p
{
border-style:solid;
border-width:15px;
}

The border-width property sets the width of an element's four borders. This property can have from one to four values.

Examples:

border-width:thin medium thick 10px;
- top border is thin
- right border is medium
- bottom border is thick
- left border is 10px
border-width:thin medium thick;
- top border is thin
- right and left borders are medium
- bottom border is thick
border-width:thin medium;
- top and bottom borders are thin
- right and left borders are medium
border-width:thin;
- all four borders are thin

Note: Always declare the border-style property before the border-width property. An element must have borders before you can change the color.

Default value:	medium
Inherited:	no
Version:	CSS1
JavaScript syntax:	object.style.borderWidth="thin thick"

The border-width property is supported in all major browsers.

Note: No versions of Internet Explorer (including IE8) support the property value "inherit".

Value	Description
thin	Specifies a thin border
medium	Specifies a medium border. This is default
thick	Specifies a thick border
length	Allows you to define the thickness of the border
inherit	Specifies that the border width should be inherited from the parent element

CSS tutorial: CSS Border