Actually, the brown recluse's venom does not contain acid. It contains a rare protein that destroys tissue cells, causing necrosis (dead tissue, which, if not treated properly and quickly, gets really bad bacteria in it and rots, causing further serious damage).
https://www.sciencedaily.com/releases/2013/08/130829214742.htm
Brown recluse aren't very large spiders, really... This one below is a "large" size, although they can get a bit bigger (notice the fiddle/violin shape on the back). Unlike a lot of spiders, their eyes are set in 3 pairs. (Although, I'm inclined to say that if you can count where their eyes are at, you're way too close!
)
One of our kittens got bitten on the neck by one, once. It causes a large, deep, nasty lesion, depending upon the severity of the bite. She survived, but she has a large circular scar on her neck from where the bite was.