The image I sent was an outlet receiving 1/8th of the total flow of a
shower, in sandy clay soil (soil type and quantity of water affect size of basin). The system is called a "branched drain" system, the pipe branches and the flow is diverted to several locations.
With a temporary system like yours you'll probably not want to spend the time to divide the flow to many locations, which is why you can temporarily have a big basin to ecologically dispose of the water.
I would split it once though, so you don't have all the flow going into one spot. We have several resources on-line for the branched drain system
here, and in my book
The Water-Wise Home I have step-by-step design/build instructions- but I don't think you need that level of detail for the temporary system.
In terms of pipe size, if you use low flow fixtures I think 1.5" would be fine for the whole system (though not up to plumbing code- most states require 2" for
shower pipes).
Glad this info is helpful for you!
Co-founder: Greywater Action, www.greywateraction.org
Author: Greywater, Green Landscape, and The Water-Wise Home: How to Conserve, Capture, and Reuse Water in Your Home and Landscape