"Is there a thin pen?"
If I were to say a skirt is ほそい, I would be saying it had a slender shape (thin) If I were to say a skirt is うすい, I would be saying it the cloth it was made from was lightweight (thin)
ほそい is used for shape: slender, thin, fine; and to mean there is little of something (e.g. food, volume, etc.)
うすい is used for thinness in the sense of substance, so is used to describe things like soup (meaning watery), color (meaning pale), taste (meaning flavorless), probability (meaning low), etc
Hopefully that doesn't make it more confusing :/