In a Reporting Services paginated report, text boxes aren't just the stand-alone boxes on the report design surface. Every cell in a table or matrix (a tablix data region) also contains a text box, which can be formatted in the same way as stand-alone text boxes.By default , text boxes are a fixed size. You can set options that let a text box expand or shrink based on its contents. These options correspond to the CanGrow or CanShrink properties in the Properties pane.
To Allow a Text Box to Grow or Shrink
Right-click the text box and click Text Box Properties.
Click the General tab.
To allow the text box to expand vertically based on its contents, select Allow height to increase.
To allow the text box to shrink based on its contents, select Allow height to decrease.