Add size prop naming guidelines

[joshblack]

Part of github/primer#6732

This PR updates our component prop naming guidance with the canonical size prop conventions from the API audit.

Changelog

New

• Add guidance for named and numeric size props.

Changed

Removed

• None.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; this updates internal skill documentation and has no public-facing impact.

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Latest commit: e38c930

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Pull request overview

This PR updates the internal style-guide documentation for component prop naming by adding canonical guidance for when and how to use a size prop, aligning with the API audit conventions.

Changes:

• Adds a new section recommending size (not variant) for visual scale.
• Defines the standard named size scale (small/medium/large, defaulting to medium) and when to extend to xsmall/xlarge.
• Documents when numeric sizes are appropriate (e.g., avatars/icons needing precise dimensions).

Show a summary per file

File	Description
.github/skills/style-guide/docs/component-prop-naming.md	Adds size prop naming conventions, including named scale guidance and numeric-size guidance.

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