Thanks Steve

We don't currently to transaction log backups - everything is in simple recovery mode and we rely on nightly backups as our current DR solution. So I don't know what log backup sizes are! It sounds like it would be worth switching to full recovery mode and configuring transaction log backups on production for a few weeks before setting up AlwaysOn - that would give us an idea of the log back up sizes with the added bonus of understanding the disk space requirements.