A unique constraint shouldn't be over 8000 bytes per row and should only use the first 900 bytes. So that the safest maximum size for your keys would be:
create table [misc_info]
[id] INTEGER PRIMARY KEY IDENTITY NOT NULL,
[key] nvarchar(450) UNIQUE NOT NULL,
[value] nvarchar(max) NOT NULL
Note: The key size can't be over 450 characters.
If the key's size is over 450 characters then switch to varchar instead of nvarchar (e.g. if you don't want to store characters from more than one codepage) then that will be increased to 900 characters.