This is a bit difficult to answer as Talybont North rooms actually come in three different sizes, whereas Taly South rooms are all the same.
Taly North houses A-G (with the exception of house E I think - not really sure how that house works but apparently it's a really good one to be in!) are all "older" houses (meaning that they're a bit grottier - if you're in house B you will definitely be dealing with mould issues). They are made up of 5 medium rooms and 1 small room. Don't even bother going for the small room. Imagine being able to touch all four walls from standing in the middle of the room without even stretching. It's not worth it. (My friend had to sleep with his legs up on the windowshelf cause he couldn't even fit in the bed). The medium rooms are okay. The beds are the same size as the "large" Taly North rooms, but the rooms are a little bit thinner. The biggest benefit to the medium rooms is that there's no wardrobe at the end of the bed, so tall people can hang their feet off the bed no problem without constantly kicking the wardrobe (: The other consideration with houses A-G though is the kitchens. They're really long and thin, so it's hard to move about when you have housemates cooking/hosting pres and you just want to get to your cupboard or grab a drink. Not the best for parties. But, then again, if you want parties you want Taly south.
Taly North houses M-Y (idk why half the alphabet is missing either) are all the "newer" houses. They have slightly bigger bedrooms with a different layout to houses A-G (you can find pictures of all the rooms on the Cardiff website), but otherwise I think they're pretty similar (bed width/length is the same I think). Again, the main annoyance is the wardrobe at the end of the bed which sucks if you're tall (you can't move the wardrobe). I can't speak for houses A-G because I didn't live there, but most of the beds in houses M-Y have drawers built into the frame, which is great for storage. But don't rely on that because I saw some in Taly North that were just a dodgy frame with no storage. (PS, rooms at the end of the corridor are like 1cm bigger than the ones in the middle of the corridor). The best thing about houses M-Y are the kitchens. They're big square rooms all the appliances around the edges and the table in the middle. Much more spacious than houses A-G and much better for pres and socialising.
I would say if you're 6 foot then Taly North small rooms are a big no. Then it's down to personal preference between the medium (houses A-G) and large (houses M-Y) rooms. Personally, I'm 6ft too and was in house Y. I had to deal with the annoying end of bed wardrobe (if you see the pictures on the Cardiff uni website, you'll see what I mean) but I would say that it was definitely worth it over having a medium room in one of the older A-G houses.
And yeah Taly south beds are wider (they also have better and bigger bathrooms).