Probably you won't want to do larger than 12x16" and eventually when (and if) you want to paint large paintings out of door you might want to get an easel that can more easily handle it such as a Julien French Easel, a Soltek Easel, or a Gloucester Easel besides one of these pochade boxes. But for getting around the city something light and easy is going to make a difference and you'll want to get out and paint more if you don't feel like a mule hauling around a lot of equipment.

http://www.allaprimapochade.com
The company Alla Prima Pochade makes an 11x14 "Yellowstone" which can handle paintings up to 18" high, it's 7 lbs. but there is a lite version which will also hold 18" high but weighs 4 1/2 lbs.

http://www.openboxm.com
The other company with similar products is Open Box M. They make a 12x16 box (recommend getting the Lightweight Kit regardless of the size you end up getting) but their 11x14 will hold panels up to 20" tall. Their 10x12 box will hold paintings up to 18" so even that is plenty big enough. The smaller boxes have less room to mix color on palette but keep in mind they are lighter too.

http://www.judsonspleinair.com
Another company is Judsons Outfitters, they are famous for their boxes too, they're called "Guerrilla boxes", you might want to buy a tripod from them. I was able to get a Bogen 3001 tripod on ebay which I'm quite happy with and Open Box M sells Bogen tripods also.

And here's some all around general information
http://www.linesandcolors.com/2008/08/17/pochade-boxes

The above is in consideration of purchasing a medium sized box, if you intend to stick to smaller sizes then there are two good choices. For doing 6x8 and 8x10's the "Cigar Box" which can be bought from Judsons Outfitters is a good choice and the "Prochade" from Artwork Essentials will handle 6x8, 8x10 and 9x12's and is very small, light, convenient and endorsed by Kevin Macpherson.