Will my SIM card from my 8 work in my new 13 promax?

Physically, yes.

Assuming the iPhone 13 is not carrier-locked, or is carrier-locked and the SIM is from the carrier holding the lock, yes.

Some carriers do charge a cellular plan transfer fee, though.

...and as anecdotal information the SIM I have in my iphone 12 providing 5G is the same physical "triple size" SIM I had in my Samsung S5, then in my iphones 6S and 7 on 4G.

O2 my operator simply upgraded my account, same SIM.

Did you see my reply?

You need a new contract for 5G, not necessarily a new SIM.

Although with Verizon's reputation for being awkward it would not surprise me if they insisted in taking control-freakery to a new high.

No, I don't need a new "contract" for 5G with Verizon. All I need is a new SIM card that supports 5G in it's settings. I have a 11 Pro on Verizon right now and have a gifted 13 Pro Max on order. I can move the 4G SIM over, but if I go into a 5G area they advised that I would NOT get 5G service as my account would not be configured for it using the older SIM card.

I don't have to sign a new contract. It's the same service with Verizon, with the only difference being your SIM is configured to connect to the 5G towers. Now, they WILL charge you a $35 fee for activation since it is a new phone (just one more money grab by them).

In fact, the doctors that have gifted me the iPhone 13 Pro Max just upgraded to the 13 Pro Max's themselves. It did not take a new contract, they simply paid cash for the phones, got them delivered with new SIM cards and activated them. They didn't have to sign a new contract, lock themselves into a time period of anything.

No "argument" to it. I'm simply telling you what applies to Verizon. In fact, even current AT&T plans don't require a "new contract" as 5G is part of all newer plans.

In fact, direct from the AT&T website is this:

All of the newest AT&T data plans now include 5G access at no extra charge. (AT&T may temporarily slow data speeds if the network is busy). If you have an older AT&T plan that doesn’t include 5G service, you’re going to want to switch to one of these newer plans.

The same applies to Verizon from what their official representatives have told me. Newer plans include 5G service, and it's simply getting a new SIM with them to enable it or activating the eSIM on your new iPhone 13. No "new contract" is required. Now, as to other carriers I cannot speak.

Yes

Size wise yes. 5G wise (according to multiple Verizon reps) no. To be able to get 5G speeds, you will need to obtain a new SIM from Verizon or utilize the eSIM in the 13 Pro Max.

It's too near Christmas to argue.

Have a good one. 🎄

Give it a rest.