If you're including a full range of scaling options (lengthening bodyparts, adding simple bulk etc) in the rigging then to a degree you only really need to get reasonably close to an average set of human proportions. The end user can thus tweak anything they don't feel is working for them.
I don't have a specific scientific or artists reference in mind (I tend to work with references of specific people for both paintings and 3d morphs), although I'm sure someone can point you in the direction of some.
As a base shape you'll probably need to decide for yourself what body type will work best with the rigging as an average in order to permit a good range of scaling and morph variations. This is always assuming that your plan is to create a figure with such versatility in mind - not everyone agrees with this approach as it takes an awful lot of time and planning and there are limits to just how much you can do with one figure. I like the versatile figure concept though.

One approach I might opt for is to try and find a willing individual in your local area to act as a living reference - someone who generally fits the description of 'average male' (whatever that is, you decide - you can look into the various scientific studies that have been done if that appeals to you). I'm imagining someone who takes regular care of their physique at a gym (fit but not overly muscular) and who is fairly proud of their body such that the idea of being 'digitised' will appeal to their vanity. I've often found such willing individuals to sit for portrait studies - not for money but just because it intrigued them. It's worth a try IMO.

I've never made a 3d figure and only made morphs for very specific projects I've worked on, so I'm just sounding ideas here. But I do know that reference is vital to achieving realism and what could be better than a living reference.
With such a living reference you could directly measure and study bodyparts and joints in various poses and as they bend - achieve as accurate a recreation as you like. The problem with just working with data compiled by someone else is that it can be ambiguous what exactly they were measuring. Working only from photos has problems associated with camera angles and lens distortion.

Just an idea.

 

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