First, I doubt that the piping in a residence is large than 4". And if you are so uncertain about the size, I am equally uncertain that your present abilities are adequate to do the job properly.
I use something like this wheel mounted on a 7" portable disc grinder. Dangerous!!!
I wear two heavy coats, goggles plus full face shield and heavy hat for personal protection. A helper with bucket of water is advisable and watch the area for fire for 45 minutes after the last sparks are created. The abrasive cutting is the best for old cast iron as the snap cutter may not break old the pipe exactly where desired.
Did I mention that this is dangerous?

Here is a link that might be useful: Cutoff

OUCH!!
Because I'm not at home and didn't get the OD of the pipe before posting, I get questioned on my abilities.
OK...
with a snap cutter, you wrap a chain around the pipe and tighten the cutter until SNAP and your done and you use a 7" grinder on a stinking pipe spraying "brown" stuff everywhere and then watch it for 45 minutes for a fire. I'm so glad I mentioned this. I will go out and buy a 7" grinder when I get ready to do it.
Thank you!!

Haha, I'm with busdriver on this one.

You're the one who said you'd done your homework. Maybe not enough. Calling a plumber when the time comes might be a good decision.

I had to cut 4" XH cast iron pipe in my basement and the best tool I found was a 4 1/2" angle grinder. It was small enough to fit in the relatively small hole I dug around the pipe, but cut through the iron pipe fairly quickly. If you have clearance to work all the way around the pipe you should be able to cut it in about 10 minutes.

I don't remember the exact cutting wheel I purchased, but it was a standard item at Home Depot. I saw a lot of sparks, but there isn't that much that can catch on fire when you have a hole dug in the dirt.

Bruce

Oh well looks like the op went back to u-tube and I hate to pass this up. Hey bus,tell us about the day you figured out you should have a helper with water standing by. HEE hee hee

I'm still sold on the snap cutter. the pipe is a few inches from an 1875 concrete basement wall. just gonna snap it, sleeve it and move on. I think I'm qualified to wrap the chain around it and turn a wrench. :) maybe I will check and ask which way to turn it

Hacksaw, 15 minutes and you could have been done by now.

It is not unusual to find cast iron pipe that is far too hard to cut with hacksaw.

I've used a Sawzall with a grit blade. Takes a while but relatively safe.

pinkelephant-
I do 99% of home projects myself, but, for this, I'd call a plumber. A plumber will make short work of this and probably cost less than buying tools, etc. Unless you're looking for a justification to buy new tools...:-)

I've just used a sawzall with a metal cutting blade. Quick and easy-- or perhaps I jet got lucky, as I don't do this often. Cast iron cuts pretty easily. No need to shower yourself and the area with sparks from a grinding wheel. A hand hacksaw with a coarse blade and a bit of elbow grease will also work for a 1X project.