I don't think Wyches changed with the last box still 25mm, I have heroes on 32mm but thats personal choice (might be required, I mean Lilith came on a 32mm so edging bets also it feels like any model with 2 or more wounds has gone to 32mm regardless of the standard troops for their army)
I also have put my Reavers on a medium oval to make them more stable and my Hellions on 40mm to make them more stable as well. Also put the Talos on a medium to large oval for look and stability.
What I've done so far with base sizes.