Hi Shiv_emf
Not all ".lib" contain the true size of the cell (i.e. x by y). Some use placement track. So the area of an inverter can be 4 placement track but the true area can be 1.245um2.
Secondly, you are wrong to say that "exact length is unknow here". Doyou understand what exactly is wireload model? Wire load model will give C, R and even length. An area/unit length can be defined in the wireload model. This is not always the case in all wire load model, but is not uncommon.
Regards,
Eng Han