Automatically pick a variable type big enough to hold a specified number

Boost.Integer already has facilities for Integer Type Selection:

boost::int_max_value_t<V>::least

The smallest, built-in, signed integral type that can hold all the values in the inclusive range 0 – V. The parameter should be a positive number.

boost::uint_value_t<V>::least

The smallest, built-in, unsigned integral type that can hold all positive values up to and including V. The parameter should be a positive number.