300 character maximum for fields while importing BAI file

Hi, we are having trouble bringing in more than 300 characters for a field when importing using a BAI transactions application. We have tried various substring logic on both the import format and via data mappings, but both seem to cut the value off at 300 characters. When looking at the previous demo that covered this, it appears a file-based source was used. Can we do the same while using a BAI transactions source app? Thanks!