I have 4 GS6000 two of them are printing 1/8 or 1/4 bigger than the size shows in PhotoShop. Does anyone have any idea why?

First make sure there really is no invisible spacing around the images that are loaded to ONYX, Colorburst, whichever you use.
Is the media you are printing being fed to a take-up-reel, and are you using a tension bar? These all are likely causes. This problem is printer side I'm positive. I have 2 Epson GS6000 printers myself, and this happens often where after printing for a few days the sizes might start increasing.
You may need to go to the printer settings:
PAPER SETUP - PAPER FEED ADJUST - LINE FEED ADJUST
*It will then print some measuring lines that in my case are always close to 19 1/2 inches.
*You will also see on the display a option to adjust numbers
*If the sizes on both marks aren't exact 19 1/2 inches each, you need to add or subtract that amount, and then convert it to millimeters. This is the number you need to enter into the printer.
Press Enter on the printer to submit.
The next test print you do should be exact size you need.