Previously, the preview image size was calculated proportionally to the page size and you had to have a border of the same proportions if there was no background. It was confusing and inconvenient when users typed in text that went outside the limits – previews would get distorted.
Detailed instructions were placed in a web-to-print help article.
A recent change in the processing algorithm allows to calculate the proportions using all objects in active layers for the page. The previews come out without distortion even if some text breaks the boundary.
The theory and the practice
It is possible that some web-to-print templates will not produce correct previews. Most likely they will produce HUGE preview images or fail. We will spot unusual error rate, but would like to hear from you of you notice anything unusual.