Hasbro have confirmed that they will be releasing a 3.75" Galen Erso in their 5POA Star Wars range. Galen will come packed with his Mouse (MSE) Droid. The info was released via Mads Mikkelsen on his official Twitter feed. Mads appears to have been sent a packaged figure as he tweeted the packaged image in hand rather than a promotional image.
Where the figure fits in the release schedule is unknown, but is expected to either be included in (or land after) wave 3 which contains the likes of Bodhi, Admiral Raddus and Jyn in Imperial Ground Crew disguise.
For any 6" collectors eyeing up the MSE Droid. This looks to be the newer and therefore smaller sculpt and will be too small scale wise for the 6" scale - you will need to look out for the older mode as reviewed HERE. There is of course still hope we will see Galen in 6" form and I think a 6" MSE Droid has to come along as some stage.