Another vote for 6x9... yes, I have 4x5, and 5x7 but as David G. noted, size can be an issue, especially when transporting on airplanes these days. I had a Century 2x3 which sold me on the concept- but not the camera. Then i lucked into a very cheap Galvin 6x9 which is ultra light and must have the largest lensboard ever for a 2x3, which adds appreciably to the applicable lens bank. I next bought a Linhof but it was too heavy and seemed very large (front to back) when folded; the Linhof back was also heavy. Then Jim at Midwest asked if I had ever tried a Horseman. So I ended up with a mint VH and have lived happily ever after. Reports to the contrary, the build quality is equal to the Linhof I owned.
The Horseman is relatively light and compact, crucial for a camera that actually has to be carried on my person. It accepts long lenses, which I prefer... I shoot a 240A f/9 Fujinon on it at infinity and can get close enough for any form of intimate landscape. There are a wide variety of specialized boards available, both extension and sunk; the 240 is mounted on a very slight (less than 1/2" extension). I shoot 6x9 and 6x7 backs...
Although many lenses will cover 2x3, you have to ask "are they sharp enough?" Unlike 4x5 and up, you WILL be making enlargements from your negatives. Some 4x5 lenses that were always decent enough in that format do not pass muster in 2x3. You will need to choose carefully, study Kerry Thalman's website. Also, focus becomes much more critical - get the best magnifying focuser you can find, otherwise you'll be saying unkind things on the light table.
If I always shot near a car, nothing less than my Canham 5x7 would do... but that excludes far too much of the world.
Your mileage may vary.
Russ