A 36 to 40 inch length deck is appropriate. Most skaters can learn on a 38” length deck. You should be able to stand with feet spread apart at shoulder's length on this board and have your feet positioned over the trucks. If your feet are on the kicktail or nose, then your board is too short for you.
Sliding decks can be purchased from a variety of manufacturers. A brief non inclusive list in alphabetical order includes: Faltown Skateboards, Gravity, Lush Globe, Pocket Pistols Cliff Slider, and Rayne.
Your trucks should be conventional geometry trucks such as Indy 169's or Tracker sixtracks.
Your wheels should be in the 60 to 65 mm size range with a harder durometer. The easiest wheels to slide are greater than 97A in durometer. Examples of such wheels include ABEC 11 No Skoolz, Gravity Sergio Sliders, Lush Cannonballs, and Powell mini logos/cubics. These wheels have the least grip and are therefore easier to slide and harder to carve. You can slide lower durometer wheels in time with the appropriate speed and skill.
My current setup is a Pocket Pistols Cliff Slider Deck with Indy 169's, and ABEC 11 No Skoolz 60 mm 88A wheels. I have no problem performing Coleman slides on flat asphalt or gentle slopes with this setup.