I just found answer here

http://social.msdn.microsoft.com/Forums/en-US/0c5a284a-8f4d-41a4-94cb-605dd2923e89/tablix-control-alignment

"If I understand you correctly, you want to keep report control’s width same to original size even if you insert some columns. Basically, the control will increase the width to adapt the new column automatically, this is by design, to work around the issue, you could drag a table with just one column into report body, and then go on deleting the detail row and footer row. Now you have only one textbox of this table. Increase this text box to the width you want to set, and then drag your original control into this table’s textbox. In this way ,you could insert other columns, however the width of the control keep the same just like your desire, it will not increase to adapt the new columns."