Dimensions are entities comprising text and geometry. They can be created using the tools on the Draw>Dimension menu. Dimension Settings are stored as Master Tool Settings.
You can set a default palette for all new dimension entities using Design Options, although you can subsequently change the palette as you can with most other entities.
You can use the Entity Inspector to modify the settings of an existing dimension.
Tools which affect dimension entities include Edit - Tidy - Dimensions and Edit - Tidy - Autosize Text.