SimsVIP has posted about a mod uploaded on ModTheSims. The Add Any Lot Size mod allows you to save and place new lot sizes into your game town.

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EA adds new community and/or residential lots in every expansion. Unfortunately the sizes of those lots can be completely random. Or even if they’re not random, they could be 40×30, which is not available in the World Editor.
This script mod will allow you to add any size (from 1×1 to 64×64) to the World Editor from the cheat console: