Open the dimension for editing and select the extension line. Then in Shape Style, change the End Arrow size. Repeat for the other extension line.
Click on the picture.
Extensions.gif
If you have more than one dimension that needs fixing, fix one of them and then get the Eye dropper tool from the toolbar, sample that dimension and then click on the others that need to be fixed. To prevent new dimensions from having the too-long extensions, get the Dimension tool, press S and sample the updated dimension. This will then make the settings for subsequent dimension in the document.
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