The Dimension lines panel has settings for changing the Color, Linetype, and Lineweight of dimension lines only. You can also change the Baseline spacing of the dimension, which is the gap between the dimension lines of the baseline dimension using its respective field, and hide the left or right sides of dimension lines using the Suppress checkboxes:

Similarly, the settings in the Extension lines panel will affect only the extension lines and you can use its settings to change the Color, the linetype of extension lines 1 and 2, and the Lineweight, and hide the extension lines using the Suppress features. Extend beyond dim lines is the tiny line segment length shown in the following screenshot, and you can choose to give it a different length or make it zero. Offset from origin is the gap between the extension line and the actual geometry as shown in the following screenshot, which you can control from the Offset from origin field:

If you want a fixed length for your extension lines, then check the box that says Fixed length extension lines and then type in the length of the extension line. By default, the length of the extension line will change with respect to the position of the dimension. So, if you move your dimension far from the drawing the extension line will be longer and if you keep it close to the drawing it will be shorter.