I would like to know what do you recommend as an ideal value for the surface width per unit for LID implementation for bioretention systems specifically?

From the help file - Surface Width Per Unit

The width of the outflow face of each identical LID unit (in ft or m). This parameter applies to roofs, pavement, trenches, and swales that use overland flow to convey surface runoff off of the unit. It can be set to 0 for other LID processes, such as bio-retention cells, rain gardens, and rain barrels that simply spill any excess captured runoff over their berms.

Thanks for your answer!

But I am confused by this article about "LID usage editor" where it says:

"The width of the outflow face of each identical LID unit (in ft or m). This parameter only applies to certain LID types such as Bioretention cells**, Green Roofs, Porous Pavement, Infiltration Trenches and Vegetative Swales, which use overland flow calculations for the unit's surface runoff. Other LID types, such as rain barrels simply spill any excess captured runoff when the barrel height or berm height has been exceeded.

If a width is provided is provided for the LID (not to be confused with the subcatchment width), it is the width perpendicular to the direction of flow over the LID's surface. It is combined with the LID's Surface Slope and Surface Roughness within SWMM's overland flow routine to find a runoff rate (in the same way as is done for regular subcatchment overland flow).

This parameter is ignored when the slope or the roughness is assigned zero for the surface layer within the LID Control Editor. In this case any additional flow beyond the berm height is converted to surface overflow."

https://support.chiwater.com/78103/lid-usage-editor

This "Surface Width per Unit" is different from the other software?