10, 11 or 12 inch. Anything bigger feels cumbersome to me.
Depends in the usage scenario.
If I wanted a laptop to use when on the road, then I would get an 11" or 13" MacBook Air (or Retina 13" MBP). If I wanted to use the laptop mainly at home, then I would get a 15" MacBook Pro Retina (as I already have done)
15", anything smaller is not an option for my eyes.
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I use ARD to control my Mini on my 15" Powerbook and the 1080 resolution is already/almost too small for my eyes.
13" MBA is ideal for work away from home, I already have a 27" iMac at home for heavy work, and an iPad for entertainment on the move.
13" is the ideal choice for me, I sold all my MBPs just because the 13" MBA is such a fantastic portable machine.