I have been thinking about this issue and been looking at how other languages, and specifically how Python addresses this need. A good big number class is based on mantissas and exponents that are unlimited. Both are based on a high speed arbitrary precision integer with wicked fast multiplies to include FFT multiplies if the precision is high enough and divide and conquer methods for medium precision. Addition is usually accelerated via some very well placed assembly using add with carry instructions. An example of this done right is the mpz integer capability in the gmp library.
Once you have a high speed integer capability, creating the floating point functions is relatively easy.
I have been using a library for Python called mpmath this is outstanding and a good example of doing it right. It has been in development since at least 2002 with much of the development funded by Google during Google Summer of Code grants. With mpmath I have been able to compute bernoulli(6000000) in 4.2 hours (numerator has 33274145 digits!) on my 2013 MacBook pro. BTW, making it multiprocessing was easy.
In any event this capability could be developed for Xojo with a good c-based plugin for big integers and the floating point code in Xojo itself. Now that Xojo has a pretty good compiler it should not be necessary for all code to be in a plugin. If python can create a fast capability, then Xojo being compiled surely should.
So in my opinion, why limit the size of the mantissa and exponent? Just my two cents.