You can edit the dimension extension lines by double clicking on the dimension and then selecting the extension line. Change the End Arrow: value to change the length and Start Arrow: to change the space between the extension line and the point you're dimensioning to.
Screenshot - 3_13_2015 , 4_16_17 PM.png
Screenshot - 3_13_2015 , 4_16_55 PM.png
Screenshot - 3_13_2015 , 4_18_00 PM.png
Also take a look at the drop down for the end style. Notice there are several straight line options. If you've got the first one selected, it'll be different from the last one in the list.