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You are generally free in motor distance and height over the sheet.
The default settings in GroundControl are 2978.4mm ~ 9.77ft, for how far the motors are apart and 463mm ~1.52ft for the height over the top of the sheet. I think they reflect the dimensions of the original frame.
It was found that a ~12ft top beam lets the MaslowCNC perform better in the low corners and many users went for the wider top. I had to go with ~11.5ft because of the 2 walls my frame is fit in.
Increasing the height could help if you have issues in the top centre.
An ‘ideal ratio’ was discussed but with no conclusive result. I attempted a most ‘symmetric’ approach based on the dimensions of a ply sheet.