When you select Dimension / Style / Modify the following dialog box will appear.

The following are descriptions for each setting within each section tab.

Lines and Arrows tab

Dimension Lines
Color: Color of the dimension line.
Lineweight: Sets the width of the dimension lines.
Extend beyond ticks: Distance to extend dimension line beyond the extension line. Example above.
Baseline spacing: Spacing between baseline dimensions.
Suppress: Suppress means disappear. You may suppress individually or both.

Extension Lines
Color: Color of the extension lines
Lineweight: Sets the width of the extension lines.
Extend beyond dim. line: Distance to extend extension line beyond the dimension line. Example above.
Offset from origin: The distance between the object and the extension line. Example above.
Supress: You may suppress individually or both.

Arrowheads
1 ^st : Sets the style for the symbol inserted for the first extension point
2 ^nd: Sets the style for the symbol inserted for the second extension point and for Diameter and Radius.
Leader: Sets the style for the symbol inserted for Leaders.
Arrow Size: Sets the Size of Arrowheads. Example above.

Center Marks for Circles
Type: Select None, Mark or Line Size: Specify Size. Example above.

TEXT tab

Text Appearance

Text Style: Select which text style to use for the dimension text. To create or change a style, select the [ ] button.