Hello,

I will be happy to assist you with this matter.

I believe the Excel Header character limitation is set by the Microsoft Excel. You can test this by performing the following steps:

1. Open Microsoft Excel
2. Insert a header
3. Add 255 characters onto an Excel Header
4. Click Save icon

Performing thses steps will give you the following message:
"The text string you entered is too long. Reduce the number of characters used."

It appears that 253 character is the limitation set for the Microsoft Excel header. 

Please let me know if you have further questions regarding this matter.