Difference between IOB and IOB2 format?
The difference is not related to the length of the named entities. Rather, it deals with how two adjacent named entities of the same type are labeled.

In IOB1 (IOB), B- is only used to separate two adjacent entities of the same type:

Today    O
Alice    I-PER
Bob      B-PER
and      O
I        O  # or I-PER if pronominals are being tagged
ate      O
lasagna  O


In IOB2, all entities begin with B-:

Today    O
Alice    B-PER
Bob      B-PER
and      O
I        O  # or B-PER if pronominals are being tagged
ate      O
lasagna  O


See Wikipedia