First, place the ruler along the bottom edge of the directory and measure it's width, then place it along the left edge and measure it's height. Multiply the two numbers and there you go :icon_twisted:
Alternately if you're just looking for the total size of the files within the directory...
A combination of file.GetFiles() and FileInfo.Length() might work for what you'd need. Using GetFiles to get a list of files within the directory itself and FileInfo.Length to get the size (in bytes) of each file and summing them up would get the total. You might need to do some recursive pathing if you want to include the sizes of all subdirectory content as well.
Hope that helps :) If it's not what you were looking for, perhaps a bit more detail on what specifically you needed would help someone come up with the best solution for you.