How to fix width of texbox ,which have lengthy data,upon postbox ?

Use the style parameter with a width property within the tag. Here is an example of one from within this very page;

<textarea class="RateComment" rows="5" cols="60" style="width:285px;float:right">

UPDATE: He is using a textbox(not a textarea) and has % as width. Fixing it to 285 would always have this width. It looks like he needs to avoid widening of the page based on long text entered into the textbox whose width is set in % such that it occupies all the available space while loading. (Issue crops up when length of text inside the texbox drives the width and not the page avaialble space)
[Update2: I know, the intent was to show the availability of the style element, nothing else. Cheers, Davie.]

You may use max-width property
Write something like this in your CSS file:

.textbox1
{
  max-width: 300px;
  width: 100% !important
}

where, textbox1 is the id of the textbox.
you can replace 100% with whatever you have specified in your code.

It works on IE6+ and Mozilla 3.5+ browsers. Have not checked for previous versions.
Check it out and let me know if this it what you wanted. :thumbsup:

Cheers!
Ankur