I think your problem might be that you're using a data source that's limiting you to 255 characters.
I'm attaching a workbook with a single worksheet which has a caption that displays all 510 characters in a field.
The data for this started out as a CSV file with 2 text fields, each with well over 255 characters. Only the first 255 characters get provided to Tableau from the CSV data engine, so that's the limit Tableau can show. I built the caption field by extracting the data and then concatenating the two text fields; since Tableau's TDE data engine isn't limited to 255 characters the resultant field contains all the characters from the 2 original fields, and Tableau is happy to show them all in the caption.
If you're using Excel, Access, or CSV as your data source one approach is to split the long fields into multiple short fields, extract the data, and then recombine the fields. Another approach would be to use the TDE API and create extracts directly; this has the advantage of not polluting the extracted data with the original split fields.
If you're not using Excel, Access, or CSV, or you're still not getting more than 255 characters in your caption from a field that otherwise displays more than 255 something else is going on that will require more analysis.
Interesting, thanks for sharing Chris!
actually I'm using Excel but later it will be a MS SQL-Server.
I'll try an inform you about the result.
It's not the easiest way but it still works with the extract
You're welcome. I think once you move to SQL Server you'll be OK.