Comfortable seating for non-giants is about 17". Antique chairs are usually under 16". Giving that you are giants (kidding) I think that may work for you, but any guest that are shorter may not be able to sit comfortably.

I just stacked some books on a chair and my feet won't even touch the ground at that height. I'm short though... 5'3".

Thanks, @littlemonkeys3. I will have some standard height chairs on the other side of the table that will work for those guests.

I wonder if kids are generally comfortable at benches (even high ones) since even regular chairs are quite high for them. We don't have kids yet, but we will, and we have lots of children in our lives.

Well kids like to squirm and kick their feet so it's probably not a problem for them at all.
I think as an adult not being able to touch the ground or at least rest your feet on something (like when on a bar stool) is super awkward, but like you said your guest have chairs. I think you should add a few books to standard chair to a measurement of 21" and see how it feels. I'm sure it won't be a problem for you. One thing to think about though is how much space you will have between the top of your legs and the underside of the table.

Could it have a foot rail?

@littlemonkeys3, I will try your experiment!! @palimpsest, that is a great idea. We could definitely build a foot rail!

re: normal height chairs on the opposite side of the table. Imagine everyone sitting around the table... shorter people in the normal chairs, taller people on the bench. I'm picturing the people in the normal chairs feeling like little kids with the taller people sitting even higher than normal on the bench. (Plus, your table height would need to be taller as well.) The more I think about it, everything needs to be the same height proportions... if the bench is higher by 3", the chairs and table height also need to be higher by 3". Foot rail for shorter legs.

I was about to write exactly what lawjedi just wrote. You'll have a bunch of tall people sitting like judges on the bench, and a bunch of short people sitting like that character Lily Tomlin used to play ( the little girl in the giant rocking chair). I think this is not a good plan.

It could easily look like George Bailey sitting in Mr. Potter's office....if you've seen "It's A Wonderful Life." Remember, the table has to work for everyone :)

Do you have room to build the 'back' of the banquette over the radiator? Maybe vent it out the top?

Is it possible to swap out the radiator to one that is smaller and would easily fit under a normal sized bench?

@lavender_lass, I should test this out. I'll stack books on one chair, have my tall husband sit there, and then sit across from him in a normal chair and see how it feels. (Maybe we can break out our best George Bailey/Mr. Potter impressions too - "Merry Christmas, Mr. Potter!" "And a happy New Year to you - IN JAIL!") There isn't room to build the bench in front of the radiator, unfortunately.

@laughablemoments, that's a great idea, and I was only trying to avoid it because I have no idea how to swap out a radiator. I'll look into it further to see how involved it is. Maybe it's not as complicated as I'm thinking. This room is quite cold in the winters because 1) it's over a crawlspace, 2) the large windows all around the room are in really bad shape, 3) this was an addition, and not a well-insulated one at that. We are considering running radiant floor heating in this room, and I plan to also replace the windows. Perhaps that will be enough to heat the room? Then we could lose the radiator altogether, or at least get a smaller one.

@littlemonkey, this would be my only dining area and I think I'd go crazy eating at bar height all the time. I could build a sideboard over the radiator and have a very small (2-top, maybe?) table and chairs in the room. That might work, assuming I then find a space for a dining table in the next room.

Many things to think about...

I am just trying to imagine what sitting on that bench would be like when the radiator came on. I would think it would be uncomfortable to have the extra warmth since it would need to be warmer than the room for the radiator to work.