My camera is very difficult to properly hide for timelapses. About 5 inches long and 2 inches wide (it's a video camera that I use for photography). I'm trying to solve a couple of issues in the way of efficiently using it as a surveillance camera: the vibrating noise it makes, and the shiny surface on the camera that may reflect light.
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Hiding the camera body is not really a photography subject.
Hiding the lens in such a way that it can see the subject without the subject seeing it has some degree of optical and photo graphic relevance. In all cases you conceal it in some way so that the lens sees as much area as it needs to do the job you need to do. The absolute minimum hole size needed to see through will depend on field (or angle) of view and distance of the lens optical centre from the hole (assuming you don't add an external lens nearer the hole). The minimum possible required hole size can be determined by placing a mask (black paper or similar) with a hole in it against the lens front surface and seeing what size you need. You may be happy with some vignetting around the edges depending on location and application. As you move the mask away from the lens surface you will need a larger hole to achieve the same result. The front of the lens will be a certain distance from the lens optical centre and if you go as far away again from the front of the lens you will need double the hole diameter for the same effect. As area increases proportional to diameter squared, doubling the distance away will increase the area by a factor of 4. One way to make a lens which is close to a hole less visible and/or to not need a small hile is to mount the assembly in a larger hood which keep light from illuminating the lens surface directly - ie it only needs to "see" light which is part of the image it is recording and any other light falling on it will make it visible without any gain in picture achieved.
One way to get a hood and distance effect is to view the scene in a mirror with the camera at right angles (perhaps pointing downward) with the whole assembly matte black inside so all you see is a "dark hole. The mirror is arranged to only allow light which is being "imaged" to get as far as the camera, with all other paths from the target terminating on matte black angled surfaces with no light falling on them. Obviously you need an outer object which does not look strange when it has a dark light absorbing cavity in it. This may be easier than expected. Various items of furniture may have nooks and crannies and holes and shafts in them in an informal way, and just by looking around a typical room you may well find something which meets this need.
If you have a choice of cameras, then a unit with a physically small sensor will also have a small lens diameter and depth for equivalent field of view. If you are prepared to have a small aperture (large f-number) you can use a "pinhole lens" - although these are usually true lenses but just very small and high f.
However, given that you can buy literally thousands of small cameras specifically made to be used as hidden devices, at prices starting at close to $0, it seems likely to be more useful to use one of them unless your camera has some overwhelmingly special capability
ALL these are hidden cameras.
From here BUT there are hundreds+ of such sites available.