Will having a splenectomy leave a big scar?

It depends. Splenectomy can now be performed safely via minimally invasive laparoscopic surgery, leaving a scar only 1-2 inches long. When done via the traditional "open" approach, the scar may be 6-8 inches long on average.
Depends on surgery. Laparoscopic -camera surgery- scars might only be a couple inches max. Open procedure scar is bigger. Your surgeon will perform the type that best suits you.