It would be most useful if you provided an SSCCE that shed some light on the following:

• What do you mean by "screen size"? See Tk::Widget. This has a number of *width and *height methods. The units of the values returned are either in pixels or millimetres.
• What do you mean by "text area"? Tk::Text? This has a -width option, which you can query. The units of the values returned are in characters.
• What do you mean by "label"? Tk::Label? This has a -width option, which you can query. The units of the values returned are dependent upon whatever is displayed in the widget.
• What geometry manager are you using for your "text area" widget? Tk::pack? Tk::grid? Other?

If you're using Tk::Text and Tk::Label, and the labels only display text, and all of those are using the same font, and your geometry management is set up such that Tk::Text resizes when its parent does, you could possibly bind a <Configure> event (see Tk::bind) to a callback that sets the width of Tk::Label widgets to be one quarter that of the Tk::Text widget.

That's the simplest scenario. You may need additional calculations if there's padding or spacing involved. If the fonts are different, you'll need additional calculations to handle that: see Tk::Font.