memory card size

There is no way to specify the configuration, only the amount. Though its likely it would be 2x16GB modules.

Unless your old iMac is a 2017 model or newer, the Ram in it will not work on a brand new iMac. The RAM modules are physically different. So it won't fit in the slots.

I recommend you purchase only 8 GB of RAM with your new 27" iMac and then upgrade to whatever you need by purchasing two 8 GB modulates or 16GB modules from MacSales.com or Crucial.com. That will cost you less than ½ of what Apple will charge.

Remember, RAM is not user-upgradeable in any 21.5-inch iMac made after 2011. What you buy today is your "forever" RAM unless you resort to expensive professional installation. Apple won't do it, and even some Apple Authorized Service Providers won't tackle that difficult job.

If you want to add RAM later. your only the 27-inch model as our wise Old Toad suggests.

RAM is interchangeable between some models.

ie a 2017 iMax 5K uses the same RAM modules as a 2020 iMac 5K: DDR4

A 2015 iMac also uses the same RAM as a 2013 model.

But as mentioned a 2013 iMac uses an older RAM type, DDR3 which is physically different to DDR4 (the edge connector has the slot in a different place), so will not actually fit into the slots for DDR4.

OT is 100% correct, please follow his advice.

Unfortunately RAM from different Mac models are not interchangeable.