I've read that we currently believe the expansion of the universe is decreasing over time from the current ~72km/s/Mpc toward an asymptote of ~45km/s/Mpc.
The size of the observable universe is a consequence of the expansion of space, and that due to the expansion: every observer has their own cosmological horizon, where relative to them any mass would be moving away at the speed of light, and accelerating.
Supposedly, the expansion rate is proportional to the mass within the observable universe, and as the mass density lessens the expansion rate of your universe is lessened...toward said asymptote, which is presumably when our local group has collapsed in on itself and is the last thing left in our observable universe.
All that said, if the expansion rate decreases over time, then the observable universe would be growing. I.e., at the same 13.8 billion light years; things wouldn't be doing the speed of light yet, relative to us. So the size of the observable universe would also be expanding toward an asymptote. Assuming the ~45km/s/Mpc is correct... The observable universe would be growing from a 13.8 billion light year radius to ~6666.6 Mpc = 21.7 billion light years.
I however, assume the universe is infinite, and that the microwave background is much more than microwave and made of highly redshifted light from the infinite galaxies beyond the cosmological horizon. I also assume that dark energy/matter is a dense ether of low energy/frequency photons permeating the infinite universe, and I assume these photons interact with each other and spawn mass into existence, eventually forming huge gas clouds, that become new galaxies, birthing additional mass into our observable universes, and possibly (likely in my opinion) at a rate that keeps the expansion nearly constant... Hubble's constant.