There is, theoretically, no limit on the text size by itself. But, the code used to display the scrolled text has potentially some limits (see below).

Anyhow, the principle is to render the whole text with a negative offset and then clip it to the window of the text widget. This is definitely not suited for a very huge text, and whole code should be changed...

So, there is a hard-coded limit in text.c:get_lines_needed():
"t_rect.h = 32767"
If the font height is 10 pixels, it means a limit of 3276 lines of text (if I'm not wrong).

Could you please do a small test: put that number to 1000, for instance, play the nethack minigame, and check that the text quickly stops to scroll ?