How does one change the default varchar 255 of a column when importing data from Excel to Sql Server using Import Export Wizard?

You may need to write your own query in the wizard and cast the columns as varchar(1000) - I'm not 100% sure that will work. ALternately, how big is the sheet? The wizard only "looks down" so many rows (IIRC 10000 by default, it's a registry setting) to determine data type and width. Re-sort the excel file so that the longest AlternateName value is at the top.

I had the same problem and I figured out that one workaround is to sort rows which are large and cause problems in Excel document according to cell length from Largest to Smallest because Import Export Wizard only checks first 200 rows.

After rows are resorted Import Export Wizard detects bigger length and change Source column data type to LongText.