Well, I've found the cause, so now the only question is what to do about it. I can slice small objects without any trouble, but not my biggest. I designed something almost to the full length & width of the stated printable volume (220mm apiece because this is an Ender 5 Pro; the object is about 216.9mm long & wide), and Cura has default margins of 15mm at all four edges that it refuses to use, making the printable area 30mm shorter in both horizontal directions than it's supposed to be.

I don't see a way to turn that off, but I do see a way to lie to Cura and claim it's really 250mm long & wide so it'll let me have back the area it stole and actually use the 220mm I'm supposed to be able to use. But I don't know how everything will react to that. I'll do a short test object as soon as possible. If that doesn't do it and there's no way to just turn the margin off, I'll have no choice but to redesign the object, which would be very irritating.

(...and then console myself about the purchase of a larger-than-average printer and not getting to use its claimed size with the knowledge that a smaller one would still have the same issue anyway.)