The footprint of the current 11" model could not be made smaller - with the current design they would have to sacrifice the full size keyboard and/or shrink the touchpad, both of which would be highly undesirable.
It isn't a question of wanting a 12" that is the same size or an 11" that is even smaller than the current model. Apple is not going to make a new 11" model, they are making a 12" model.
My best guess taking in all the rumours related to this product:
- An 11.88-12.2" 16x10 retina display
- All new Fanless clamshell design that follows the iPad Air design language
- 3-4 Lightning connector ports (adapter dongles to be sold separately for Thunderbolt, USB 3, Gb Ethernet, SD card, etc)
- Full size backlit keyboard (same size as current 11" model)
- Redesigned pressure sensor "buttonless" trackpad with normal tactile click feedback
- Headphone jack
- MagSafe 2 power charger
- Intel Y-series Broadwell CPU's with Iris pro or its successor integrated graphics
- Bluetooth 4.0
- ac Wifi
- Base model 4GB RAM (BTO up to 16GB)
- Base model 128GB PCIe superfast SSD (BTO up to 512GB)
- 10+ hour battery life
- OSX 10.10
With this list, if it is a good representation of the machine we are going to get, will be an insta-buy for me and a whole host of those who have been waiting for retina to come to the ultraportable line of Apple's laptops.
I would imagine that this single machine will replace both the current models and be branded as the Macbook Air with Retina display. The current non-retina models with stay in the line-up for another year without any updates, in a similar fashion to the 13" non-retina MBP, but will be discontinued with the next update of the retina MBA.
Apologies if this is off topic and not what you were after at all. In answer to your original question, I would rather have a 12" the same size as the current 11", simply to keep the full size keyboard and trackpad.