Forced external convective cooling

I have what must be a stupid question about cooling by external forced convection. I'm trying to model a metallic object that is locally heated on one surface and that will be cooled on the other surfaces by convection of air, and by radiative heat transfer.

I'm trying to model a case where there is an air current flowing in the room with a speed of U0 that is causing this cooling. In the heat-transfer physics mode, I choose "convective cooling" for all non-heated surfaces. In the heat flux settings, I set
- external forced convection
- plate, averaged transfer coefficient
- U_ext = U0
- external fluid: air
- pressure 1atm
- external temperature 293K

so far so good - but Comsol asks me for a "plate length" and this isn't explained anywhere in the user's guide as far as I can see. Is the plate length just the dimension of the object that I am modelling? Or is it something else?

thanks for an answer
Martin