In Windows SharePoint Services, the description field of a workflow history list is truncated when the number of characters used goes beyond 255.
This behavior is a by-design limitation. Windows SharePoint Services implements a restriction on the number of characters that can be used in a multiline text box. The reason is that the OLE property stream doesn't support more than 255 characters. Having more than 255 characters would cause problems demoting properties into documents.

As a side note, this issue can be reproduced in other areas from within SharePoint with similar results. This means that this behavior is not limited to the description field of a workflow history list but other areas of SharePoint as well like multi-line text fields in columns in a document library, adding a new link through the SharePoint interface to an existing site etc..