How to display a block width of 640 pixels on a mobile device?

Question

css

Hello. I have a website, 640px wide.
I would like the site width to be the same 640px in mobile design, but just a larger font (font sizes are not a problem), and mobile would show 640px regardless of its extension as 100% full screen width.
How to tell mobile device to show pure 640 pixels as 100%?
PS I can't make the design stretchable, because my html is immediately broken there.
I just specify the font increase in css depending on the width of the device, but I don't want to change the width of the site itself.

Answer 1

<meta name="viewport" content="width=640">