Very large directories can have their sizes returned as floating point
numbers, eg `1.0034576985781e+14` from the box API.
Before this change this would fail to parse as an int64.
This change parses the size as a float64 instead which will be
perfectly accurate for sizes up to 2**56 which is about 9 PB.
It is unknown whether box themselves use a float64 as an intermediate
representation in the API or not - it seems likely.
Fixes#2261
