The size of the vector glyphs has a tremendous effect on the Vector plot's readability. VisIt uses an automatically computed scaling factor based on the diagonal of the bounding box as the size for the largest vector. You can augment this size by entering a new scale factor in to the Scale text field. It is also possible to turn off automatic scaling by turning off the Auto scale check box. When automatic scaling is turned off, the vectors in the Vector plot are the length specified in the Scale text field.
If you want each vector to be further scaled by its own magnitude, you can turn on the Scale by magnitude check box. When the Scale by magnitude check box is off, all vectors are the same length as determined by the automatically computed scale factor and the user-specified scale.