Use Type properties and the Text tool to change the size of dimension, tag, and element text.
This video demonstrates the following:
All text in Revit is an annotation element of one kind or another. This includes text used in dimensions and tags. The size of annotation elements is relative to the scale of the view. As the scale of the view is changed, the relative size of the text is changed. For example, when the scale of this view is changed from 1:100 to 1:200, the text appears larger. When the view scale is changed back to 1:100, the text appears smaller. The text changes because the printed size remains the same, relative to the view scale. The printed size of the text is defined in the Type Properties of the text family.
Select a text element, and click Edit Type to view the Text Size parameter. The size listed here is the printed size for the text. To change the size of text placed with the Text tool, create a duplicate family, and make the necessary change to the Text Size parameter. In a case where the family name does not indicate the text size specifically, you do not need to duplicate the family. In this case, the name of the family type contains the text size, so it is better to duplicate the family first.
It is important to note the text size change in the text family only affects text placed using the Text tool. Text in elements, like dimensions or room tags, needs to be changed independently to match the text family.
Dimensions are also system families, and have a similar type property that controls the text size. Tags, however, are loadable families. The text size in a tag is controlled by the label size in the family. The tag must be edited, and the label family must be changed in the family and then reloaded into the project.
If you need consistently-sized text, the text size needs to be controlled by family type parameters and needs to be changed in multiple families and categories. In most cases, setting the text size in your template files is the easiest method of establishing the text sizes for your specific project needs.