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I'm not sure what size you are trying to get to, or what the biggest build is yet.
Here is what I think about the size. If you get over 3-4 feet in any direction you should use some supports under the outer rails. The biggest effect of a large build is going to be in the z direction. X and Y should hold much better than Z. So if you are looking to cut full sheets I think you will be fine if you cut through a little extra into your spoil board. Don't expect full sheet 3d reliefs at crazy speeds.
I tell everyone figure out the biggest piece you might cut and try it out, very cheap to resize it smaller, just a little time. But don't make it huge in case one day you want to cut a full sheet. The smaller it is the better tolerances and the faster you can cut.
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