questions about DIY WB card

Just read this short DIY tutorial for a WB card. One of the comments at the end was:

Got my North Sea D90-60 from Wilsonart Laminate a couple of days ago. Used it today calibrating my Sekonic L-358 meter. Results are stunning. My meter was off by 3/4 of a stop (overexposed.) The Custom White Color balance via the gray card helped me dial in the meter exposure. Happy camper here!

I don't quite understand this comment. How did the WB card help the user with exposure? Isn't the card something you stick in one photo to use later with the eye dropper tool to set the WB all of the other photos taken in the same conditions?

Another question I have is size. Why are some WB cards pocket sized and others rather large?

Grey cards were first used as aids to metering when you didn't have an incident meter. They reflect light the same as an "average" scene would, a level that corresponds to the 18% reflectance grey they're printed in and which light meters have typically been calibrated to. That they are also neutral in colour is what makes them useful for white balance.

However, note that the product he's using is not designed as either a photographic grey card for metering nor a white balance card; rather it's a sample chip of countertop material for interior designers.