While I can understand your dilemma, don't not do it just because surgeries can be risky. *Any* surgery is risky, whether for a human, cat, dog, or rat. If your vet is experienced, there's not a whole lot to worry about. Sure, we all chew our fingernails off, eat entirely too much chocolate, and annoy the receptionist every 30min, but I know my vet's fully capable of handling anything that may happen. My vet's assured me that she'll stop a surgery if she even suspects that something is not going as planned.
See, as that tumor gets larger, it has every possibility of attaching itself to vital organs. It can invade to such a point that it's impossible to remove later on. It could also attach so firmly to her organs that it obstructs those parts of her. Some tumors also develop blood vessels all through them as they grow larger, which makes it almost impossible to remove them later on. Some tumors get so entrenched in the body that they just start sucking all of the nutrients from the rat, making it difficult to keep them hearty and healthy, and keep their quality of life. At some point, it will begin to hinder her movement. If it gets large enough, it will start dragging the ground and an open sore can form.
She's in perfect health right now. There's nothing saying that when the tumor gets too big that she will be. She could have the same problem as several other older rats, where a chronic resp issue won't go away enough for you to be comfortable with surgery. In dozens of cases, spays have stopped the tumors from reappearing, or at the very least caused any new tumors to develop much later in life and were extremely slow growing. Even when the rat has already developed tumors, it's done a world of good.