What size hard drive do i need for a xbox one 2.5 or 3.5 ?

You would need to send the console to Microsoft to be repaired. The internal hard drive is not meant to be swapped out and would violate the terms of service to do so.

The 2.5 enclosure would only fit a 2.5 inch hard drive. That gets plugged into the side of the Xbox one. The 3.5 inch hard drives sit on top of the Xbox, with an external power supply like you stated. The video showed this as well. What I was wondering
is if I could use a larger capacity hard drive inside that 3.5 inch enclosure instead of the 4TB the video mentioned?

The drive needs to be USB 3.0 and have a capacity of 256GB or greater. It can be 2.5" and powered by USB, or 3.5" and powered by mains power.

The 4 main specs to look out for are: -

Capacity - Obviously, how much data it holds. It needs to be at least 256GB, but you can get 3.5" externals that are 4TB if you want a ton of storage space. 2.5" drives go up to 2TB in size.

Size - 2.5" or 3.5". If you don't want to plug the drive into the mains and you want it to be small and more portable, go with 2.5" (bearing in mind the differences in capacity and speed), otherwise go with 3.5".

Speed - Some drives are 5,400 RPM (such as the Xbox One's own internal drive) and some are 7,200. Generally speaking, the 7,200 RPM drives are faster than 5,400, so you'll shave some seconds off loading times with a 7,200 RPM drive. A 5,400 shouldn't be
any slower than the Xbox One's existing drive though.

Interface - It has to be USB 3.0, so you have no choice there!

Hope that helps!

Usually 2TB or less drives are usb powered, upwards of this size usually come with an additonal power supply that has to be used.

(2TB or less drives are usually 2.5 inch and the larger ones are 3.5 inch and need additional power to run)

I bought a Touro Mobile Pro, which is a 7,200 RPM 2.5" 1TB drive. I went with that because it doesn't require any external power - it's powered over USB 3.0 by the Xbox One - and because it's 7,200 RPM. It's pocket-sized, so I can easily take it over to
a friends' house.

Any drive that meets the minimum specs (USB 3.0 and greater than 256GB in capacity) will be fine, but here's some extra info that might help: -

Capacity - How much data the drive holds. It has to be 256GB or above. 2.5" externals (powered by USB and nice and portable) go up to 2TB in size and 3.5" externals (that you need to plug in to a power socket) go up to 4TB. There are some 3.5" externals
that have an even greater capacity, but they contains 2 drives inside them (so, 2 x 3TB drives for 6TB in total, for example) and I'm not sure how well they would work on the Xbox One.

RPM - How fast the drive spins. All other things being equal, the faster the drive spins, the faster data can be read from the drive so a faster drive will shave some seconds off loading times. A 5,400 RPM drive will work just fine (that's what the Xbox
One's internal drive is), but you can also get 7,200 RPM drives.

Size - The choice here is 2.5" or 3.5". A 2.5" drive is powered by USB so you don't need to plug it into a power socket. Almost all 2.5" drives are 5,400 RPM with a maximum size of 2TB. There are a couple of 7,200 RPM 2.5" drives out there, such as the Touro
that I mentioned, but I can't find any bigger than 1TB. A 3.5" drive is powered by external power and you can get 4TB externals that are 7,200 RPM.

Interface - It has to be USB 3.0.

Brand - I would go with any well known brand.

So, it really depends what your priorities are. You certainly don't need to get caught up in all of those specs because as mentioned, any USB 3.0 drive will be fine as long as it's got a capacity of 256GB or greater, but that info may help those that have
particular needs.