Joachim,
As I know, there is no exact limit for reading xml lines, but according to my experience, it can be extremely slow with big amounts of data. If you really need to use so big files, then I suggest to create an addon, that can handle the data files, or create a script that converts the data to a macro (if it is possible…).
Can you tell me more about the development details (you can send it in mail, if it can’t be published here…)
Gergely Fehér
Team Leader, Library Team
GRAPHISOFT SE