Generally speaking, overriding a dimension's value is one of the cardinal sins of drafting. But we know sometimes it's necessary, so we've given you a way to at least make it obvious that the text doesn't match the actual length.
Let's say I've got a 600x1 rectangle, but only an 8.5x11 piece of paper. (For all these images, click on them to see the full-size version.)
But that means that my dimension showing the length of the rectangle isn't correct anymore. So I'll override it, but I'll also add a Jog Line to the dimension line. This shows that the dimension doesn't represent a real-world distance.
Jog Lines couldn't be easier to use. They're on the Dimensions panel of the Annotate tab, and you simply select the dimension and pick a location. You can grip-edit its position later, and you can set the size of the symbol (as a factor of text height) in the Dimension Style Manger.