Thumbnail sizes are determined by the template, ensuring that the thumbnail size does not break the design. This is crucial so that your images look correct on all internet enabled devices.

For a quick comparison, see this FAQ:
Main Page Image and Thumbnail Image Size per Template

If you want larger thumbnail images on your chosen template you will need to request a custom change. 

Please review our custom work policy here:
Custom Work Policy

If you want to try another template for a different thumbnail layout, see this FAQ:
Artwork Thumbnail Layout on Various Templates

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