When you need to change the size of the textbox you usually change it's size in the property grid.
But what if this size still remains the same when you preview the report? Most probably the values of CanGrow (more details about CanGrow property here: CanGrow (Render)
) and CanShrink (more details about CanShrink property here: CanShrink (Render)
) properties on the Property Grid are set to true. In this case, the size of the textbox will be selected automatically when running the report. If you want to set a certain size of textbox, you need to set CanGrow and CanShrink properties to False.