Constant MAX_SPAN_LEN 

const MAX_SPAN_LEN: usize = 306;

Defines the maximum possible length (in bytes) of a Span printed in the friendly format.

The way I computed this length was by using settings that would produce the maximal amount of text and using a negative Span with each unit set to its minimum value:

SpanPrinter::new()
    .padding(u8::MAX)
    .designator(friendly::Designator::Verbose)
    .spacing(friendly::Spacing::BetweenUnitsAndDesignators)
    .comma_after_designator(true);