Depends what species they are. Some stay quite small, some get much larger.
I have a baby Pelinobius muticus that's going to be quite large one day. They're one of the larger species, and I can expect around an 8" diagonal legspan at maturity. (Diagonal legspan is the measurement from the foot on a front leg, to the foot on the opposite back leg, with legs stretched out).
She's also female, so she'll be bulkier. The males of most species tend to be lankier, while females are bulkier. She just molted again last night, so I'm excited to see her growth once her exoskeleton hardens.
My other species are a decent size as well, but the Pelinobius will be the largest of them. You can also get dwarf species that have a 2-4" diagonal legspan. There's all kinds out there.
As for personalities, in my experience, they totally do. Others might argue with me on that... But once I've had them for a while, I get a pretty good idea of their personalites, temperaments, etc.
All of my current ones have different personalities. Several years ago, I had three rescued spiderlings of the same species. It didn't take long to get to know them, and each of the three was different.