Question from a customer on an internal mailing list this, succintly answered by RTF Template God, Hok-Min
Q: Whats the upper limit for a sum calculation in terms of the largest number BIP can handle?
A: Internally, XSL-T processor uses double precession. Therefore the upper limit and precision will be same as double (IEEE 754 double-precision binary floating-point format, binary64). Approximately 16 significant decimal digits, max is 1.7976931348623157 x 10308 .
So, now you know :)