Can’t help with that question, sorry - I usually end up in row 6 on Alaska, and they don’t allow bags on the floor in bulkhead seats.
^^^ Hmm, then I’ll probably have to buy one to find out . . .
I have a large Longchamps tote. It fits fine under any seat. I don’t have one with a horizontal zipper…but it’s pretty big to begin with!
The one with the horizontal zipper can turn into huge – much bigger than the large tote. I don’t have one, but I wish I did.
@HouseChatte - when the bag is expanded and completely full, it’s pretty hard to fit under the seat, although if the contents are squishy enough, it is feasible depending on the airline.
It has the shorter leather handles similar to the other Longchamp bags, but also a detachable wide shoulder strap.
^^^I have the same Longchamp bag that expands when you unzip the middle section. I use it for traveling all the time. It must be 8 or 9 years old and it still looks great. It is a dark purple color and I had the leather flap that goes over the top zipper monogrammed so it would be easier to spot. I almost always check it because it is packed full.
It is back in stock at the stores:
http://m.shop.nordstrom.com/s/longchamp-le-pliage-expandable-travel-bag-21-inch/298032
The link gives dimensions.
There is also this tote:
Coach is changing its corporate name and ticker symbol:
^^ what?!?! Say it isn’t so!! My wonderful H just bought me 5 new Coach bags: 2 totes, a cross body and 2 different sizes of clutches all leather. Oh well, I love them all, they look timeless and the coach logo is a small metallic thing anyway so you can’t tell.
I just saw the news about Coach in the video news in the elevator at my office building. Seems to be a terrible idea as both Coach and Kate Spade are such recognizable names and marketed as such.
I am going to email them today actually. Why change a venerable name, makes no sense to me at all.
Oh they will keep the brand names! Those names are worth major $$$$. It is the house of fashion that now owns the three brands that will get a new name. 
So there are two different expandable Longchamp bags. Decisions, decisions. Thanks for all the info on fit and sizing!
Yikes about Coach. I’m no marketing genius, but it’s almost as though they’re undermining their own appeal by trying too hard.
We have coach stock so my H feels compelled to buy me a Coach purse every now and then. Good thing he’s got good taste and I like coach purses. I’ve found my purses have gotten progressively smaller over the years as I evolve towards being more minimalist, and several older (larger) coach purses are going up on the Ebay chopping block this month.
I use a small wristlet that has all my gym info (and just fits my phone and key fob):
and I currently am using a purple purse that’s small but fun and holds my pac man wallet:
https://watchcove.com/products/coachww_f55736_saddle
My favorite of all time coach purse is this teeeeeeeny little studded one that I use for going out on dates with H (no, that’s not mine up for sale on ebay, lol, I love it and I’ll keep it for a very long time):