I despise when lines of text extend across the whole viewport, regardless of device. Especially on external monitors, this creates bad UX/UI, and it’s exhausting to read this content. It harms readability.
You’ll notice that popular websites fix line lengths, often placing text within neatly-organized blocks. 40-80 characters per line seems reasonable, toward the lower end for mobile, though 50-70 is ideal. This means a font size between 12-18 points for body text.
This requires an absolute width, at least on large screens longer than 1000 pixels, for the section containing body text via CSS.
Know that anything looking odd versus typical expectations can harm UX, which increases bounce rates and therefore affects SEO, so consider fixing the width of your body text on large screens.