If pelvic exam finds big mass, what are the chances I will need a laparotomy and not a laparoscope?

Probable. It depends on the type of mass & size. A large solid mass which appears malignant may require a laparotomy. A large cystic may sometimes be done laparoscopically. It depends on the malignancy risks, your surgeon's skill level, & the hospital's equipment. Your surgeon will discuss the options with you before the case & obtain your informed consent. Good luck with your surgery.
Need more info. If pelvic exam reveals a mass, imaging with ultrasound, ct or MRI together with exam findings will suggest if surgery is warranted and which type of surgery is best. Minimally-invasive surgeries are available for many large masses, even malignant ones. Patient health (and wishes), nature of the mass, expertise of the surgeon and local resources are all factors to consider in surgery planning.
Depends. If the characteristics of the pelvic mass is cystic and appears benign on ultrasound or MRI or any type of other diagnostic tool then laparoscopy maybe attempted. If there is any chance that there is a premalignancy or malignancy to be encountered, a laparotomy is a better idea.