What is the ideal width of my newsletter for the import?

For newsletters, the norm is a width of 600 to 620 pixels.

Wider newsletters are not recommended. Many people view their newsletter in a preview next to a number of other columns (with, for example, a list of folders and headings of other messages). This does not leave much room for the newsletter itself.

By sticking to a width of 600 to 620 pixels, you prevent your subscribers from having to scroll horizontally. Many experience this as annoying.