Linda,
Both pipes have the same inside diameter (1 1/2"). Technically, it's one size too small for modern washers, but it'll probably be OK. What you need to do is cut the steel pipe a few inches away from where it threads into the tee. Then get a 1 1/2" fernco adapter to clamp on the steel pipe and the 1
1/2" PVC you'll be installing. Get a PVC trap that glues together and is as thick as normal pipe (schedule 40), and put a standpipe in the trap that reaches to the top of the washer.If this design overflows, or gives other trouble, then you'll have to increase the sizes to 2" and replace the cast tee with a PCV teewye.