Smallest Cold Frame size?

I have a 4 foot by 6 foot wide bed next to my lawn and I would like a small cold frame.  Could I build a 2 foot wide frame and then plant something next to the frame that only gets 2 feet wide.  It would be exposed in this area and I am thinking an evergreen on the North side of it would help shield.  Or should I not worry about a plant screen and make it four feet wide.  I am not needing to plant alot in it and would prefer the smaller size but that might not be large enough to keep the plants inside warm?

Beth, my concern from reading, is not keeping things warm, but preventing them from over-heating when the sun shines. Even in my relatively moderate climate, the fellow I knew who had a 4x8' cold frame, had jars of water inside to help accommodate the extremes. I can remember in my youth, on days when we had mixed cloud and sunshine, my dad charging out to put a sheet over his cold-frames when the sun unexpectedly came out.

I am not sure if I will use it during the summer as I have plenty of raised beds I am trying to keep filled.  Moreso in the winter just having a few greens for smoothies.  So not sure if a 2' X 6' frame is big enough to stay insulated or maybe size doesn't matter...

What kinds of highs and lows are you looking at? Where I am, the limiting factor would be sunshine, not cold, to grow greens.

If your limiting factor is temperature, being willing to put an insulating blanket between the plants and the glass at night would be a huge help, but that becomes a twice/day task.

You imply that the cold winds come from the north, so yes, putting some insulation on the north, +/- east and west walls would help, and the insulation could be either or both plants or something like recycled bubble wrap covered to keep the sun off it. However, if you use plants, you don't want something that you can't keep trimmed away from the glass.

Putting a dry-stacked wall of recycled bricks or similar  on the plant side of insulation will help turn some of the sunshine into radiant heat, but it will radiate in all directions without something insulative on outside of the stack.