I tried to use the blob.h and blob.w commands to determine the edge length of a square.
I think that he gives me the edge length in pixels but I need it in cm, can I convert it or is there a command that outputs it directly in cm?
The camera has no way to know what the size of the object is. You have to write code that converts pixels to mm using a scaling method. It should be a linear scale for your object so just measure how many pixels equals how many cm and multiply the pixels by that ratio.