10E500?

[Snorre]

Hello mpowell,

You cannot configure the Prime to go up to 1e+999.
The floating point number's exponent is three decimal digits wide, i.e. 1000 possible values: ranging from 1e-499 to nearly 1e+500.

Check out the constants MINREAL and MAXREAL (You'll find them also within the [Units] menu) and notice the differences in HOME (decimal floats) and CAS (binary floats).

[EDIT] Despite these limits are by far big enough for all physics, You're free to program some kind of big floats (handling mantissas and exponents separately) for Your special needs, e.g.
bigfac(n):=alog10(FP(sum(log10(k),k=1..n)))+"e"+IP(sum(log10(k),k=1..n))
to get the faculty up to 10001 (or even higher when summing by a program loop).

Greetings