Septal perforation about 2-2.5cm - what are the chances of saddle nose?

First, how did you acquire the perforation? That's something that needs to be understood and discussed with an experienced nasal surgeon. This is important to ensure no ongoing process is present which could make progress in size.
Second, the amount of remaining dorsal cartilage strut determines your tendency to a saddle nose deformity. The amount in the caudal strut would affect tip support. (as general principles)
Consult with a surgeon is evaluate the above issues and determine if you are at a higher than average risk for future saddle nose deformity.

Hi there! Septal perforations can be challenging to manage. A 2-2.5 cm perforation is substantial in size. The fact that you're asymptomatic is a good thing. The most important ways to prevent a saddle nose from developing are consistent follow-up with an ENT or facial plastic surgeon familiar with septal perforations and hydration of the tissues. If the perforation continues to change over time, then that is concerning and may require further workup or surgery. If it remains stable, then routine follow-up and hydration may be all that is needed. In the meantime, using saline gels and sprays, in addition to humidifiers at night, can help with creating stability of the perforation. Hope that's helpful.

That’s a pretty big perforation. It would only cause a saddle nose if the perforation is close to the bridge. If it is low down it far back you don’t need to worry