So, as I understand, there is no any strict recommendation that HtmlWidth should be less than PageSize.width, it just impacts the scale how PDF is generated (the more HtmlWidth the smaller scale). Correct?

No. htmlWidth value should take into account only the HTML content. Let’s say you have a 1000px-wide HTML table. If you define htmlWidth=850, the not fitting 150px of the table are going to be cut in the rendered layout. The visible 850px will be mapped to the target paper width, independently if the format is A1 or A4 or A6.