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I printed something big, and it went off the edges of the build platform graphic in MatterControl seemecnc version 1.2

I suspect that the image/model/whatever is for the Orion build platform, which is obviously smaller than the Max V2.

What file should I use and where should I put it to fix this?

Other than the watermark logo the bed image is generated dynamically based on the bed size (found in 'Slice Settings' > 'Printer' > 'General' > 'Bed Size'). On the Orion bed you should see a 7.5CM radius (each line is one CM) and on the V2 a 14CM radius.

Note: Prior to 1.2 changes to the bed size didn't immediately update in the preview window (you had to toggle between a different part to trigger the reload).