Hi DeeLynch,
The front rack mount kit bracket is about 7 inches long and the rear rack mount kit is about 7.75 inches long. These are estimates and not the exact values, but very close. I have attached the installation guides for both. As you can see in both documents, you can install the chassis safely up to 4 inches back or forward, so it is not recommended to put the chassis on the very ends. The kits come with the screws, so use as many screws as possible for maximum stability. You may be able to stabilize it with some additional support, but what I have said above is what is recommended by National Instruments.
I hope this answers your question, but let me know if it didn't or if you have any others! Have a great day!
Chris R.
Applications Engineer
National Instruments