long drain pipe leaking
One thing to consider is that metal pipes are often grounded and may even have electrical connections using them as a ground (think appliances). Running a section of PVC will interfere with this so be certain that nothing is connected or run a seperate grounding line back to the other pipe or to the nearest feasible point.

Using the larger pipe will be better especially if it has solids passing through it (kitchen sink for example), don't be afraid to use online stores to find exactly what you're looking for. One option is to buy from the hardware store and return the fittings if they don't fit (although if you've used solvent weld etc then that may cause issues but nothing stopping you from sliding them over and checking before installing).

Bring your tape measure to the store!