What is the string length of a GUID?

It depends on how you format the Guid:

• Guid.NewGuid().ToString() => 36 characters (Hyphenated)
  outputs: 12345678-1234-1234-1234-123456789abc

• Guid.NewGuid().ToString("D") => 36 characters (Hyphenated, same as ToString())
  outputs: 12345678-1234-1234-1234-123456789abc

• Guid.NewGuid().ToString("N") => 32 characters (Digits only)
  outputs: 12345678123412341234123456789abc

• Guid.NewGuid().ToString("B") => 38 characters (Braces)
  outputs: {12345678-1234-1234-1234-123456789abc}

• Guid.NewGuid().ToString("P") => 38 characters (Parentheses)
  outputs: (12345678-1234-1234-1234-123456789abc)

• Guid.NewGuid().ToString("X") => 68 characters (Hexadecimal)
  outputs: {0x12345678,0x1234,0x1234,{0x12,0x34,0x12,0x34,0x56,0x78,0x9a,0xbc}}

36, and the GUID will only use 0-9A-F (hexidecimal!).

12345678-1234-1234-1234-123456789012

That's 36 characters in any GUID--they are of constant length. You can read a bit more about the intricacies of GUIDs here.

You will need two more in length if you want to store the braces.

Note: 36 is the string length with the dashes in between. They are actually 16-byte numbers.

The correct thing to do here is to store it as uniqueidentifier - this is then fully indexable, etc. at the database. The next-best option would be a binary(16) column: standard GUIDs are exactly 16 bytes in length.

If you must store it as a string, the length really comes down to how you choose to encode it. As hex (AKA base-16 encoding) without hyphens it would be 32 characters (two hex digits per byte), so char(32).

However, you might want to store the hyphens. If you are short on space, but your database doesn't support blobs / guids natively, you could use Base64 encoding and remove the == padding suffix; that gives you 22 characters, so char(22). There is no need to use Unicode, and no need for variable-length - so nvarchar(max) would be a bad choice, for example.

I believe GUIDs are constrained to 16-byte lengths (or 32 bytes for an ASCII hex equivalent).