It is not a case of 'how big the zoom' is it is a case of how close a lens will focus.
For example a manufacture does a lens with a zoom of 100-400mm. This is great for getting head and shoulder portraits and sports BUT as the closest the lens will focus is on 1.8m then your 75mm figure will look very small in the image.
If your looking at camera specs then you need to look for something that will focus closer than say 30cm.
The best thing to do is if you have a camera in mind, find a shop, take a figure along and a memory card and take some pictures to see how close it can focus and how much detail you can get.
To achieve the type of thing you discribe would require, I think, a DSLR and specilised lens.