Does anyone know of a good audio interface for portable use with a laptop?
 
It would need to be very small, and would only need a headphone jack. Strictly for mixing in the field.
 
Thanks for any ideas!

Here's the one I ordered:
 
http://www.audioquest.com/dragonfly-series/
 

Glad you found something. I've never heard of the product. Let us know how it works for you.
 
I didn't respond initially, because I thought every suggestion I would have would be much more than you were looking for. I was right. 

I'm pretty happy with my Focusrite Scarlett 2i2.  Although it has more features than what you're looking for.  Tempting fate, it's also just the right size to provide a flat surface for my coffee.

Slugbaby
I'm pretty happy with my Focusrite Scarlett 2i2.  Although it has more features than what you're looking for.  Tempting fate, it's also just the right size to provide a flat surface for my coffee.

I have several audio interfaces. The one I use the most is the Scarlett 2i2. I use bigger ones when I'm doing stuff with a lot more inputs and/or outputs. But for sheer mobility and mixing on the go with my laptop the Scarlett 2i2 works really well.

If all you needed was headphones (output) and no input,, all I do is leave my interface behind and use the on board audio to muck about.. never been an issue for just editing audio.
But of course you will always need good audio drivers once you get into recording and using soft synths. So I'd be interested to see if what you bought will be much of an improvement over on board audio. Without drivers I doubt it. They claim better audio quality ...hmm.
I've never experienced pour sound quality with any laptop that I could detect. I often just use my on board audio for blasting my backing tracks through the PA at gigs..
 
If I need to be recording I find my 6i6 to be very portable. It's all about audio drivers no matter how portable you want to be. Other wise Sonar will not be happy.

My two criteria are:
 
1) Much better audio (not just louder)
2) AS small as possible
 
I do a lot of mixing through my headphones, and spend a lot of time outside the house. I mean studio. ;-)
 
So I need the great quality, and the portability.
 
It should arrive on Monday. I'll report back!

Cactus Music
If all you needed was headphones (output) and no input,, all I do is leave my interface behind and use the on board audio to muck about.. never been an issue for just editing audio.

This was my first thought as well. I think a wiser investment would be sonarworks headphone calibration software. It did wonders for my AKG K240's.

http://sonarworks.com/headphones/overview/

fwrend
This was my first thought as well. I think a wiser investment would be sonarworks headphone calibration software. It did wonders for my AKG K240's.
http://sonarworks.com/headphones/overview/

+1 for SonarWorks.  I have the Super cheap ($30 at Amazon) Superlux HD 681 Dynamic Semi-Open Headphones  and the SonarWorks helps a LOT.  Don't use these for tracking as they are Semi Open and you will get bleed!
 

I should be getting my USB DAC today! Can't wait to hear the difference. 
 
+1 on Sonarworks! I use it with my ATH-M40x phones.