feat(sbb-form-field): add size s

[DavideMininni-Fincons]

Preflight Checklist

• [x] I have read the Contributing Guidelines for this project.
• [x] I agree to follow the Code of Conduct that this project adheres to.
• [ ] I have searched the pull request tracker for a Pull Request (PR) that matches the one I want to submit, without success.

Issue

This PR Closes #2545

Pull request checklist

Please check if your PR fulfills the following requirements:

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)

See Review Guidelines for more information what is checked during review process.

Changes

Changes in this pull request:

• sbb-form-field accept 's' as size value

Browsers

I tested the build on the following browsers:

• [ ] Firefox Desktop
• [x] Chrome Desktop
• [ ] Edge Desktop
• [ ] Safari Desktop
• [ ] Chrome Mobile
• [ ] Safari Mobile

Screen readers

I tested the build on the following browsers:

• [ ] JAWS Firefox Desktop
• [ ] JAWS Chrome Desktop
• [ ] NVDA Firefox Desktop
• [ ] NVDA Chrome Desktop
• [ ] VoiceOver Safari Desktop
• [ ] VoiceOver Chrome Desktop
• [ ] VoiceOver Safari Mobile
• [ ] Android Accessibility Suite Chrome Mobile

Pull request type

Please check the type of change your PR introduces:

• [ ] Bugfix
• [x] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Other information