I would also like answers to these questions

There is a physical limit to seeing things with just light. IIRC, atoms themselves cannot be seen with light unless there’s a large molecule of them. They use electrons to see them.

When you say “vision” you are probably meaning like-human vision. My understanding is that a lot of smaller animals rely more on smell and taste, than sight. Although flies can see fairly well close up. But, in terms of “smallness”, I don’t think most animals can detect microscopic organisms with sight but perhaps with other senses.