The thing you need is to make sure what things exist in your city.
To start with creating a huge playable city in WE is impossible.
Just focus on creating the biggest part you can while units can move etc.
They thing you really want is to know what exactly exist in your city.
For example.
In the end of 4151 street lives an old man with small garden and a dog.
His house ain't large, just normal.
Then
*searches and find pieces to construct your own house (there are a lot in the Hive)
creates the house, creates the garden, adds a dog and a crazy old man*
Boom!