Normal preview image size was limited to 400px by 400px.
It has been increased by 50% to 600px by 600px. Other preview sizes increased by 50% as well with Jumbo size going up to 1200x1200px.

Old preview size

New preview size

Changes

1. Any new templates will automatically get the new preview size
2. User-generated previews will be generated in the new size
3. Existing template previews remain unchanged
4. Images from orders will not change

Negative implications

Some templates with low grade images may start looking grainy when enlarged by 50%.
It would be probably be better to use higher quality images, but please, inform us if this becomes a problem.
The new preview size is experimental and will be adjusted based on user feedback.