I don't know of any 19's or 21's, but a friend of mine had (still has, in a loft he says) a 17" monitor which handled 15KHz modes with no problem.
Well, no problem except that the gap between each scanline was quite pronounced. Not a fault, just an unavoidable side-effect of dividing up quite a large screen into only 256 lines. Any 19 or 21" CRT capable of handling 15KHz modes will make this problem quite bad.
I set up WinUAE on my old desktop a few weeks ago and had the output displayed on my 19" CRT. When displaying original style scanlines, it was a bit of an eyestrain. It was only really usable when they were doubled.
I tell you what would be a good hack effort: Mashing up an existing LCD's controller gubbins to make it work with those old modes. It surely can't be beyond the realms of possibility?
Of course - you "could" use a 19" or 21" LCD TV via the RGB SCART connection which works very well. Limited resolution though.
Normal monitors (better than the Philips and Commodore ones) that can display more resolutions were the 14" / 15" NEC Multisync (which "may" also have been available as a 17" from memory) and also I actually have a wee Acorn monitor which will be going on sale soon that works with the amiga. Not sure about resolutions that it can display though.
Thought I'd try my 1200 on my Panasonic Viera 42" plasma in the living room.
I had it running through RGB Scart. Picture quality is superb. It is flicker free & looks great. Brings the games to life again !
I had tried it with the same scart cable on my ATEC 37" 1080p. The picture quality wasn't great on the 1080p & I had doubts it was actually seperating the RGB signals, looked like a composite signal.
I had the 1200 on a 1084S previously but may just put the 1084S away for storage.