Double Click the Center Mark to edit, Under settings check option 'Set Extension Individually' and drag the arrows to adjust the length.
How can the default Center Mark size be set?
NX seems to prefer ridiculously large Center Marks. I'm modifying Templates to address other issues, but I can't seem to find the default settings for Center Marks.
The customer default options for controlling most of the default settings for drafting objects are found by editing the drafting standard, and then saving the edits. Here's where the standard is located in the Customer Defaults dialog:
In the Customer Defaults dialog box, select the Drafting→General/Setup node→Standard tab.
From the Drafting Standard list, select a standard.
Click Customize Standard.
Once you enter the drafting standard settings, you can find options for controlling the center mark under the Annotation-->Centerline node.
In order to save the changes, you'll need Write permission to the drafting standards file you're editing. Or you can save a custom version of the file. Just remember to use that custom standard when you set up your template.
Here's a link to more information about using and customizing drafting standard files: