If you’re going to do it with a real penis, go for average to above average size and girth but ensure that the top goes slowly. Smaller for your first time leaves you wanting more, wanting bigger, and is generally not recommended. If you never do it again, at least you’ll have had a larger penis inside of you and won’t have the regrets of having only had a smaller one.
I tried with a dildo a few times now, can get it all the way in. I didn’t hate it but it’s a completely different feeling. Wouldn’t mind trying with a smaller sized guy someday.