Adjust file finder width configuration (#20819)

Follow up to: https://github.com/zed-industries/zed/pull/18682

This PR tweaks the setting value, so it's clear we're referring to
`max-width`, meaning the width will change up to a specific value
depending on the available window size. Then, it also makes `Small` the
default value, which, in practice, makes the modal size the same as it
was before the original PR linked above.

Release Notes:

- N/A

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Co-authored-by: Kirill Bulatov <mail4score@gmail.com>