Namaste,
Skandapurana is the largest Mahapurana. It is said that Skanda Purana has 81,000 slokas and is as large as Mahabharata (without Harivamsa). Purana scholars like Jvalaprasad Misra say that Skandapurana is not found anywhere as a single text. It is found in the form of samhitas, khandas, Mahatmyas, Sthalapuranas, stotras, etc which are ascribed to Skandapurana. But, we may not find many of them in the Skandapurana available today.
Please see page 291 of 'Ashtadasa Purana Darpana' by Jvalaprasad Misra for a discussion on the nature of Skandapurana text.
JP Misra says that Skanda Purana has two recensions - one consisting of 6 samhitas and the other consisting of 7 khandas.
Sivarahasya Khanda is said to be a part of Sankara Samhita. This is available online at the links given below.
There is an independent Upapurana (some say that it is an itihasa) named Sivarahasya (or Sivarahasya Purana). This Sivarahasya Purana has 12 Amsas and there are around 1,10,000 slokas. It is larger than Mahabharata. Ribhu Gita, well known Advaita Vedanta text is part of this Purana.
This was published from Tanjore Sarasvati Mahal Library in 12 volumes. Almost all are available online. Link is given below.
You may also refer to 'The Puranas' by Ludo Rocher (this is a part of A History of Indian Literature) for a general discussion on the Puranas and Skanda Purana. This book has a discussion on categorization of the Puranas based on three gunas as well as on the basis of five devatas (page 21).
regards
shankara