Rule of thumb: 5 - 10 m per p.e..
The length/width ratio can be calculated using the Darcy Formula. This formula determines the maximum hydraulic loading of the system to avoid overland flow. Small systems (<10 p.e.) are commonly designed with a length between 6 and 15 m.
The bottom of the reedbed can be slightly sloping (0.5% -1%).
Depth inlet: 0.6 m; depth outlet: 0.8 m. The depth of the wetland depends on the depth of the rhizosphere of the vegetation.
The hydraulic rentention time is about 4 to 5 days.

On these pages you can find a spreadsheet based on the first order plug flow kinetic model proposed by Reed et al. 1995. This model can be used to design a HSSF.
Horizontal Subsurface Flow CW's: Dimensions
Constructed Wetlands