What was the mean distance photons travelled before colliding with electrons in the matter plasma before recombination?
I have checked other answers close to this but they only mention a mean distance but not what it actually is.
Since the cat is out of the bag and I calculated this in another (non-duplicate) question, I'll repeat the calculation here.
The baryon density today is about $\rho_0 \sim 4\times 10^{-28}$ kg/m$^3$, then at a redshift of $z\sim 1200$ (just prior to recombination), the number density of free electrons (assuming a fully ionised hydrogen gas) is roughly $$n_e = \frac{\rho_0 }{m_u}(1+z)^3= 4\times 10^8\ {\rm m}^{-3}$$
The mean free path of a photon in the plasma is $1/(\sigma n_e) = 4\times 10^{19}$m, where $\sigma$ is the Thomson scattering cross-section. Thus the average photon can travel about 4000 light years before being scattered.
Since the "size" of the universe is $\sim ct$, where $t$ is the time since the big bang, and since $t \sim 300,000$ years at $z\sim 1200$, then the universe is effectively opaque to the radiation within it.