I have not read or heard that size affects the severity ( survival rate). I know a fellow whose tumor was over 7cm when he was diagnosed. In his case, he had a kidney problem and an ultrasound accidentally discovered the tumor. He had decided to remove the bladder even it was not muscled invasive and he is doing fine.
In general, statistically speaking, the severity (survival rate) depends upon how deep cancer has progressed into the muscle tissue and beyond. Statistically speaking, the deeper cancer has progress, the survival rate becomes lower. But it also depends upon if it is metastasized or if lymph nodes are positive or not.
In terms of staging, if it is metastasized in other organs, it is classified as stage IV. If one lymph node is positive, it is at least stage III. If it has progressed beyond the muscle tissue and progressed to the fat tissue, it is stage IIIA or IIIB based upon how far it has progressed in the fat tissue. If it has progressed to the muscle tissue but not reached the fat tissue, it is classified as stage IIA or stage IIB. Below is the link to the document of how the stage is determined.
emedicine.medscape.com/article/2006834-overview
Usually, a urologist recommends the removal of the bladder if cancer has progressed the muscle tissue. Because of your father's and the complex surgery (4-6 hours) and the recovery process, the urologist will discuss also the treatment with preserving the bladder.
I am sure that many people in this forum can share their experience when you need to make a certain treatment decision. Also, it is noted that there are several new treatments developed for bladder cancers in recent years that were not even available five years ago.
Regards to 9 cm size, a hallmark of any cancer is that it keeps multiplying (dividing) 7x24 without stopping so the colony of cancer cells keeps growing. A healthy bladder only makes new cells to replace old cells.
best
