there are standards:
1" normal and 1 1/8" oversize

Many new bikes have oversize fork (that is the diameter of the steerer tube)

The basic thing you have to look at is match the standard 1", or 1 1/8" (or other oddballs 1 1/4, etc), and check for the height of the steerer tube (height of the head tube). The steerer tube can be cut, and can be threaded further more (for threaded forks), but it won't fit in a longer head tube.

As I can see bianchi via nirone 7 C2C has 1 1/8" oversize non-threaded fork, with a relative short steerer tube (head tube)
You need to check the length of the head tube to be shorter than the length of the steerer tube of the fork (plus ~40mm to fit the stem)