At first:
http://www.itu.int/rec/R-REC-BT.1700-0-200502-I/en
At second:
Based on AGNUS spec
http://eab.abime.net/showpost.php?p=736465&postcount=99 - maximum amount of visible video lines is 573 (625-2*26).
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btw Amiga HW can display IMHO unlimited amount of lines - limitations is only time (i.e. number of lines will be at a cost of the refresh time and memory).