There are some antivirus for Dropbox? Why free version have so few space?

The only issue is to have files clean, free for viruses. Can I use any software?, I´d like to know how it works. thanks in advance for your answer. When I have worked before paid versions have a lot of storage space, but the free version is so far, I would hope more consideration on that, sometimes we can pay, sometimes don´t, but I still want to recommend it.