Can you set scissor to a specific size?

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Hi guys! I’m hoping to acquire a volume of interest of the trabecular bone of the vertebrae body. So basically my plan is to segment the lumbar vertebrae first and then use scissor function (set the shape as rectangle and erase whatever that’s outside the rectangle.)However I was hoping to see if I could set the rectangle to a fixed size like the brush for paint/erase, wonder if anyone knows how to achieve this? Thanksss!!