HOW: set FRAMESET "rows" propterty in .css file

frameset rows is a html element attribute, not a style property, so it can't be set using css.
You can do it using javascript.

:( damm...
but i cant use JS bcuz im using Server Based Page (ASP.NET)
any suggestions?

Admittedly I know very little about ASP.not, but I would think, that like any other serverside scripting environment it works by sending html code to the browser, in response to http requests. While javascript runs on the clientside, it's completly independent of what the server is doing, and can be embedded in the html code emmited by ASP, or included in a seperate file, just like CSS code.
But why not just set it using ASP ?.

Admittedly I know very little about ASP.not, but I would think, that like any other serverside scripting environment it works by sending html code to the browser, in response to http requests. While javascript runs on the clientside, it's completly independent of what the server is doing, and can be embedded in the html code emmited by ASP, or included in a seperate file, just like CSS code.
But why not just set it using ASP ?.
Khp is right, you can send the javascript code to the client browser, actually ASP.net sends javascript to the user all the time, you may want to take a look at the "source code" that you get when invoking an ASP with your browser to realize how it works.
I don't understand why modifing the attribute in the ASP will make your page slow, what are you trying to do?

it is the first page and in .net - the first page is always slow bcuz .net loads everything.

That should only apply to the first pageload after the .net server has been restarted or the asp file has changed, hardly anything to be concerned about, and cirtainly not something you can do anything significant about. As for setting the attribute in ASP, 13 bytes or so of extra data will probably impact load times by less than 0.001 seconds.