I'm curious why you ask?
No limit is set in Fiddler's code, but experimentation suggests that either the .NET Framework or Windows 8.1 itself only permits 30000 characters in the combobox control.
In practice, many servers will refuse requests whose headers are over 16k in length.
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Do you want to know when a feature you care about is added or when a bug fixed?
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and vote to affect the priority of the items