Ignore what Guy McGowen ...

Ignore what Guy McGowen said.  He is just advertising a unit not suitable or fit for the task you outlined. 

Lamella clarifiers are common for surface water silt removal and other industrial water clarification needs (in process and wastewater) and are not intended for disinfection.  The sizing of lamella-type depends on the silt load and you did not note that.  However, in general, you can size Lamella-type clarifier to about 160 liter per minute per sq meter (4 gpm/sq.ft.) .   Thus, about 15 sq meters.  Actual width and length (and height) will very by manufacturer.  Parkson is the industry leader in these.

Proper use of coagulants and flocculants is critical to success.  Following this with media filter is typically recommended to address floc carryover.

Good luck.