Thinking about the body...

Well, one of the biggest tricks is "how big is BB-8"?  I took a bajillion pictures of her at Celebration, and have other references, so we have a vague idea.  She ran around Artoo, and as club builders, we certainly know his diameter.

Anyway, after punching in a bunch of numbers, I came up with just shy of 20" for BB-8's diameter.  Then you've gotta remember that the builders are from the UK, so inches are probably not their first measurement system of choice.  (Just listen to Lee & Oliver try to convert).  So 20" == 508mm.  Now that's a VERY interesting number.  Basically 500mm or 1/2 meter.  Now that is the kind of number you'd expect a UK design team to come up with.  I can just hear the conversation now:

"So, how big should she be?" asks the English builder

"About so" (holding arms about so far) says the Director.

"That looks like a half metre" (notice the English spelling)

"Ok, half meter it is" (American Director spelling).

So I'm going with 500mm.  There've been lots of discussions and numbers thrown out on the forums, but the next most common value is 20", which is pretty close.  Hopefully if I'm a little small it won't be noticeable.

Ok, so you've got a size, how do you make it into a Lego sphere?  Fortunately I'd seen a link to an online tool before, and a quick Bing search found me "Bram's Sphere Generator".  That cool tool make a sphere out of 6 identical panels.  I tried a bunch of different numbers and settings and options until I found one that seemed like it would work.

Adding some orange rings got me to this sphere:

Initial Lego BB-8 Draft Test Sphere.

It's beginning to look a bit feasible!