I don't think each area has a specific dimension to it. I think it's all based of what size picture you use and where it fits itself in. Don't think you can get that specific to where you want it exactly without fiddling around with the settings and different pics.
The pics won't fit in perfectly. Goes off of these settings here -
And dependent on the size/graphic placement on the pic you use. You just have to fiddle around with those settings/different pics to get it where you want. Hard to explain. For example, if you want 1 picture for your whole background on your profile, it won't span the whole way perfectly fit in there the way you want it, without having to tile it (Both), etc.