My backyard observatory, pictured below, is located in southern New Jersey (Atlantic County). The roll-off-roof structure was built by Stoltzfus Structures of Atglen, PA. They built it at their location and delivered it fully assembled and ready to go. The interior is 16x12, which is big enough for two piers, two computer stations for astrophotography, and still some space to spare.
Below is a picture of the interior with both piers ready for some photography. The far pier hosts an Astro-Physics 140EDF f/7.5 Starfire on an Astro-Physics 900 mount with a Borg 50 acromat guide scope and a Lodestar; the main camera is a Starlight Xpress Trius H694. The near pier has an Astro-Physics Mach1 mount on it that typically holds a wider-field setup, usually either an Astro-Tech AT65EDQ (f/6.5, focal length 420mm) or a Borg 60ED with super reducer (f/4, focal length 240mm) with an Atik 460ex camera.
These three scopes give a nice range of focal lengths, roughly related by powers of two, of 240, 420, and 840mm (the 840mm comes from using a 0.8x reducer on the 140EDF), and image scales of 3.8, 2.2, and 1.1 arcsec/pixel. This provides a good deal of flexibility to handle objects of different apparent sizes in the sky, and even larger objects can be captured using mosaics.