Leto, largest possible lot is 64x64 (even in CAW when making new world). Largest residential lot in SV is 60x60. Also, if lot is mark as building, you won't be able to even try placing it there, since no icon will appear. About fence and move object, I once made a "fence" out of rocks and plants using move object, and some of them were practically outside of the lot. Once I copied it and placed it on a same sized lot, but half of the rocks were missing, without any message. There is no way that game would save your lot as bigger than it is.
You might want to recheck size of your mansion and the size of a lot you're trying to place it. It is probably what Pam said, too many objects on a lot so it became corrupted in a way.