Determine the size of a hashed varchar column
Well - it appears that a fellow Stack Exchange member (Richard Marskell - Drackir)  has figured this out here.

I've copied his answer below:


  You should use the binary datatype. You can use binary instead of
  varbinary because the hash function will always return the same number
  of bytes for the same type of hash (e.g. MD5, SHA1, etc.). This will
  cut down on the (slight) overhead required to manage a variable length
  binary (varbinary) column.
  
  In terms of what size to make it, you can run this query to check the
  length of each hash type:


SELECT  DATALENGTH(HASHBYTES('MD2', 'Testing')) AS [MD2Length],
        DATALENGTH(HASHBYTES('MD4', 'Testing')) AS [MD4Length],
        DATALENGTH(HASHBYTES('MD5', 'Testing')) AS [MD5Length],
        DATALENGTH(HASHBYTES('SHA', 'Testing')) AS [SHALength],
        DATALENGTH(HASHBYTES('SHA1', 'Testing')) AS [SHA1Length],
        /* 2012 only: */
        DATALENGTH(HASHBYTES('SHA2_256', 'Testing')) AS [SHA2_256Length],
        DATALENGTH(HASHBYTES('SHA2_512', 'Testing')) AS [SHA2_512Length];


And it should come out with this:

MD2Length MD4Length MD5Length SHALength SHA1Length SHA2_256Length SHA2_512Length
--------- --------- --------- --------- ---------- -------------- --------------
16        16        16        20        20         32             64