There are several ways to import data into Open-Xchange, but all are restricted by the fact that OX uses a SQL database to store data about its core elements (contacts, appointments and tasks). That means that the field size is limited.
If there is such a problem, the data will be shortened/truncated to field the field. The technical details vary a little:
The fields have been sized generously while still having efficient row sizes for fast SQL queries. If you encounter a problem, it usually lies with generic fields like "description" or "comment", which some other programs use to store vast amounts of data (for example a VCard representation of the same contact you just imported).
To determine whether data fits the database, the size of the database fields can be used. But two things should be kept in mind: Depending on the character set used, a character can take up more than one byte. The default character set used it a 3-byte UTF8 set. Chinese installations, for example, might use a set that takes up more bytes per character or not even UTF8 at all. Also, some installations have used different field sizes for reasons ranging from performance to missing updates (though updates to database tables are really rare).
Now you have been sufficiently warned: What follows is the data of a typical default installation. Yours might vary.