Unfortunately the Flex SDK Slider was not designed to have its appearance easily customizable through simple properties. If you want to tweak it, you have to dive in to skinning. Here's a Flex 4 example of skinning a Spark HSlider:
Don't worry , many developers feel that slider thickness is something that could be improved. There is nothing worse than unveiling an application , just to have poor reviews due to an inadequate slider. The folks at Adobe know how damaging an undersized component can be to a developers ego. So in light of this , they brought skinning of components to the forefront of Flex development. Now you can keep the functionality that your users have enjoyed for so long , and your components will always be the "right" size.
Here is a great link to follow. It shows making a slider a more colorful and engaging experience for the user