fix: respect size setting on split Button

[esclapes]

The size property was not being used on split buttons. Sofia noticed it in the Queue view in the webapp, and thought I would fix it here.

Before:

After:

Description

Motivation and Context

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style and guidelines of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have updated the CHANGELOG document.
• I have added tests to cover my changes.
• I have performed a self-review of my own code
• I have tested my changes/additions in the latest Chrome, Firefox, and Safari.
• I have commented my code, particularly in hard-to-understand areas
• All new and existing tests passed.
• I have performed the accessibility audit of my UI changes according to the accessibility doc.
• [Buffer Engineers] Someone from the Design team reviewed and approved my changes
• [Buffer Engineers] I have notified the BDS team of my changes in the #proj-design-system Slack channel