Smart Hub arrived yesterday and seems to be exactly the same size as the HH5 it replaced. They even give you a bag to send your ould router back to them free of charge for recycling, but I think I'll reset mine and get a few £ for it on an auction site.
Initial set-up was fine, though there was on annoying restriction. On the HH5, I had set-up the 2.4 and 5Ghz frequencies with different SSID name and wanted to set-up the Smart Hub with the same SSIDs and passwords to avoid updating all existing devices manually. 2.4 was fine, but for the 5Ghz frequency, the Smart Hub automatically puts a -5 suffix to the name, and there's no way you can get rid of it.
Parental Controls have a bit more fine tuning available. I can now have different profiles for my kids devices for different days of the week (e.g. give them an extra hour at the weekends when they don't have school next morning).
Overall, it seems to handle multiple devices much better than the HH5, as we experienced less slow downs than normal. My desktop computer which is furthest away from the router now gets an extra 5 Mb/s download throughput, but overall signal strength is about the same or slightly better.
Another bonus is that it reports the amount of data downloaded and uploaded by device, so no longer can the kids get away with the excuse of "I'm not downloading anything that would make the internet run slow..." In the 4 hours of normal internet use, our household had downloaded just under 8Gb, the majority of which was accounted for by our two teenagers! Good job we have an unlimited package through BT.