haha thanks so much for your responses guys. A few minutes after I had posted, I realised that length of an IC grows in proportion to the number of pins used (i.e An IC with 40 pins will be scaled longer relatively to that with only 8 pins). Therefore, generally length doesn’t have to be altered manually.
As for width, we can change it by specifying the spacing between the pins as prompted by one of the specifications (i.e 300, 600mil or so).
Thank you so much for your time guys! I have successfully customised my own IC in Fritzing.