Does it have a long, skinny body or a fat one? There is one type of brownish-black spider about that size that is poisonous (I'm in the US mountain states, warmer places may have more). It's called the brown recluse or "fiddleback," which is the one with the skinny body. A Google image search will let you know if this is one of them or not. That said, there are many types of harmless spiders that match your description, including catfaces, daddy longlegs, and wolf spiders.