The babies are very brightly colored compared to the adult counterparts. The baby one my 6 year old found was about 4 inches long and smaller around than an adult pinky finger. (no he didn't touch it, we have had all the predator talks)
Adults are a much duller color unless they have just shed their skin. And much bigger, not sure how big they get.
As for spiders there are 2 that I really watch out for. Like previously said black widows are around but you don't normally find them in inhabited places so I don't worry about them. The ones I do watch out for are brown recluse, impossible to avoid, easy to smash.... and wolf spiders- they are pretty large, not poisonous but pack a heck of a bite. I find these normally during the late spring and throughout the summer when they are most of the time carrying their babies on their back (looks like they have a huge butt, don't poke it with anything they will all fall off and run around in a large spread). The bites can easily get infected but they tend to run rather than bite.
Fact about wolf spiders: They are the ONLY spider that carries the babies on their body until the are old enough to hunt on their own. ALL other spiders fend for themselves from day one.