To handle screen size variations in mobile GUI design, follow these steps:

1. Design for multiple screen sizes: Create designs for various screen sizes, such as small, medium, large, and extra-large.

2. Use flexible grids: Implement adaptive grids that adjust to different screen sizes.

3. Utilize relative units: Instead of pixels, use relative units like percentages or ems for layout and sizing.

4. Prioritize content: Focus on essential content and features for smaller screens.

5. Hide or collapse elements: Hide or collapse less important elements on smaller screens.

6. Use responsive images: Ensure images scale accordingly with screen size.

7. Test on various devices: Test your design on multiple devices and screen sizes.

8. Use mobile-friendly layouts: Leverage layouts optimized for mobile, like single-column or card-based layouts.

9. Consider orientation: Design for both portrait and landscape orientations.

10. Use design tools and frameworks: Utilize tools like Sketch, Figma, or Bootstrap to streamline responsive design.

By following these steps, you can create a mobile GUI design that adapts seamlessly to various screen sizes and provides an optimal user experience.