Fiddling around with a service mode myself here, I found this thread over at Atari Age where a Atari 2600 author asked people to determine visible area through a program.
The results are insofer interesting in that a lot of somewhat modern crts didn’t fully display the standard NTSC 224 lines. One person’s old crt is even below 192. Assuming this is mostly without tweaking, there are some statements in that thread regarding what’s possible then. For PAL I’d say the average is as I expected - average 263 lines.