I would imagine there would be several ways to estimate it.

I'm seeing the diameter of a carbon atom (in meters) is about

1.52 × 10^{-10}m ( 0.000 000 000 154 m)And, here is a calculation for the volume of the human body (in meters), at about

0.07 m^{3} for a 70 kg person.Hmmm

So:

[tex]\frac{0.07 m^3}{(1.52 x 10^{-10} m)^3}[/tex] gives the approximate number of generic atoms.

[Alternatively, you might choose to calculate the number of moles and atoms in the body using Avagadro's number, 6.02214 ×10

^{23}. You could also estimate for the major constituents, carbon, oxygen, hydrogen, nitrogen, etc.]

To convert that back to meters, multiply that back by the size of a carbon atom.

[tex]\frac{0.07 m^3}{(1.52 x 10^{-10} m)^3}[/tex] x [tex]{1.52 x 10^{-10} m [/tex]

And you end up with:

[tex]\frac{0.07 m^3}{(1.52 x 10^{-10} m)^2}[/tex] in meters.

And, I came up with about 3 × 10

^{18}m (3,000,000,000,000,000,000 m)

That is bigger than I can count in meters, but it comes out to about three quadrillion km.

A lightyear is about 9.4605284 × 10

^{15} meters

So, you end up with about 300 lightyears.

While it doesn't quite reach across the Milky Way, it would reach a quite a number of stars.