In the Excel Add-in users guide it says the following about the size of the fields
Field data must not exceed the field size in ALM.
Only plain text and hyperlinks are uploaded to ALM. Any graphics are ignored, and formattingis removed.
The Description field in ALM is a Memo field that has no size limit in ALM
On the database side the ST_DESCRIPTION field is defined as varchar(max) which can hold a maximum of 2,147,483,647 characters.
There should not be a 255 character limitation on the description field
Check your mapping make sure that you have mapped the fields to cells correctly and that you do not have two fields pointing to the description cell.