Since you took out the balance of the account, the amount that was an RMD, isn't really important. You can answer that all of it was an RMD, or you can say part of it was an RMD, that is more accurate, but then you need to enter your RMD amount.
The trustee of the account should have told you how much you have to take out, that would be the RMD, but if you don't have that information you can just estimate the amount.
The RMD is based on your age. If you are 60 it is about 4% of the account value. If you are younger it is less, if you are older it is more. There are RMD calculators on line if you want to be precise.
One thing you should know is that, no matter what you answer, the information is only used to see if you owe penalty. These numbers don't appear on your tax return.
Since you took the full balance of the account you do not owe a penalty.
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