I have had plenty of video cards with composite T.V. out on them and they were able to output 1366x768
They may have had framebuffers that size, but your TV didn't show it – unless you actually meant Component (three coax cables, often red, green and blue)
Composite video tops out at 625 lines of data for PAL, except that they aren't all for the image – in the UK we actually used the top few for Teletext/Ceefax!
PAL allows up to 576 lines for the image, although most domestic TVs don't actually show all of them either.
Horizontal resolution is a bit more complicated as it's two analog signals, a "colour burst" at the start of the line, then brightness along the line – so there is a higher "brightness" resolution than "colour".
You can generally assume about 720 pixels per line – that's what PAL DVDs and Freesat/Freeview SD digital transmissions use.
So a reference PAL monitor has a native resolution of 720 x 576.
- Though not all analog TVs can actually show the whole thing anyway (overscan).
NTSC is worse – which may be why HD took off so much faster in the US compared to Europe.