I don't think this is possible, even in CAW you can only create this max. size. Is a 64x64 lot not big enough?
You could build a little town on it
Also take into account that the performance would drastically be impacted if bigger lots would be possible.
Even on a 64x64 lot with lots of objects, furnitures and such - plus many sims visiting, this might lead to problems.
You could make your own world in CAW and there you can place lots seated beneath the other with just one grid space so you could make several placed to look like "one".
The small gap can be filled with terrain painting overlapping to both lots (this is not possible in normal gameplay), so it looks like one.
Regards/Gruß Claudy
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