[Dream-Runner (OP)]

Assumptions:
1.   Our 3-D universe is on the surface of a 4-D globe.
2.   Gravitational wave travels within the 4-D globe at speed of light.
3.   Gamma-ray, as well as other electromagnetic waves, travels along the surface of the 4-D globe.

radius.jpg (28.41 kB . 767x476 - viewed 2236 times)
 
As illustrated above, X is the distance of the cosmic event; R is the radius of our 4-D globe; C is speed of light; ∆t is the time difference between detection of gravitational wave and gamma-ray; H is the Hubble constant (about 72 km/s/Mpc).

X – 2R*sin(X/2R) = (C+H)*∆t

On 8/17/2017, gravitational wave GW170817 was detected. 1.7 seconds later, gamma-ray burst GRB 170817A was detected.  The binary stars are 40 Mpc (130 Mly) away. 
Put these numbers together, the radius of our 4-D globe is around 1015LY (1 quadrillion light-years).

The estimate could be more accurate with more and more cosmic events being recorded.

[xersanozgen]

4B'lik dünyanın küreseli yarıçapı yaklaşık 1015LY'dir (1 katrilyon ışıkyılı).

the radius of our 4-D globe is around 1015LY (1 quadrillion light-years).

R = 13.43 G Ly