I got my a 40" Seiki Pro at work.
http://www.newegg.com/Product/Product.aspx?Item=9SIA8RW34J5238&cm_re=seiki_pro-_-2W2-000P-00001-_-ProductI am driving the monitor from an AMD/ATI Radeon HD 6450 based card via a display port 1.2 connection. The PC has a 6th generation i5 with 32GB of RAM.
1.) I chose a "light" font scaling of 125%. It really isn't needed by it added a very "smooth" character to text. 100% scaling is fine as well.
2.) After using this monitor, 40" is the smallest desktop monitor I would every want to get the proper amount of workspace.
3.) I am about 24" to 30" away and I get get the full desktop area will requiring little head movement.
4.) Altium looks great. Spreading a PCB across the entire screen is awesome. I plan on using a 24" monitor to the left in portrait mode for docking the project tabs.
5.) Given the ample workspace, having another 40" monitor would be too much for me. Another screen in portrait mode is fine.
6.) I think a 50 to 55" would actually work well for 4k as well if you could get 6 more inches away.
7.) I use eclipse based tools for programming. Having 4 code windows side by side is very nice!
8.) 3d rendering in Altium works well. It pushes the low end video card in the machine (which is primary due to the half-height form factor of the Dell). Rotating, etc is smooth except for very complicated models. A modern gaming card would be a good idea. (Workstation cards are a 100% ripoff)
9.) Solidworks and Autodesk Fusion 360 are a dream. The large workspace make working on models easy. AutoDesk Fusion 360 was a bit slow as I had all the effects turn on and the card I have in the machine is a bit underpowered.
Bottom Line:
If you have the means, go with a 40" 4k. Anything smaller (lots of 28" monitors out there" would yield little benefits as you woudl have to scale the display to see text. You then lose the workspace Given that the monitor is your primary interface to to the design space, spare no expense!
I plan on getting an identical setup at home with a new GTX970 based card.