For me 13" is optimal. I was a long time fan of the commodore 1084s monitor but with age they have become very unreliable. I spent more time repairing them then it has become worth it. The nice thing about them was that they had a case and no open frame like a standard arcade monitor where you could drop something into it or touch inside inadvertently if you were moving it.
I would suggest that you search for a 13" enclosed monitor that supports the 15.75 kHz horizontal scan rate of standard resolution arcade monitors. A standard VGA monitor does not. A little googling will lead you to some options then you can hit craigslist, ebay, etc.
I found a 13" studio monitor that supports 15.75 kHZ up to standard VGA as well and has two other inputs for NTSC and component video. I am going to hook up the cable box to it so I can watch programs on it when not fixing a board and when I am - keep the audio on the monitor to the cable box. It is nice since it is enclosed - you could put stuff on top of it and since it is studio quality it should be much more reliable.
I have a spare 13" open frame arcade monitor as a backup - if you do go this route make sure that you use an isolation transformer with it if necessary. I made a monitor test supply by gutting an old PC AT power supply metal case using the IEC AC connector for power, adding a switch, panel fuse holder and indicator light with the isolation transformer safely inside. The output is the standard 2 position power connector with a green grounding wire with spade lug to screw onto the monitor chassis. I have a huge label on this test box reminding the user that the green grounding wire must be attached to the CRT before use.
If you come up with an innovative solution, please share.
Bill