A 2TB and under will be the easiest to setup, but it really depends on how may games you have or want to have on the drive. I have ALL the US Wii & GameCube titles, plus quite a few EU titles & some JP titles on one single drive which takes up about 4.5TB, so you'd want a 5TB drive to have everything in one place on one drive. See the last link in my signature if you want to more about using drives larger than 2TB and there are a few pictures at the end of that post.
I'm considering doing the same here, full set GC and WII on my WII modded with internal HDMI (Wii Dual project).
On my setup I have a powered usb dockstation so power isn't an issue.
And I have a 8TB drive that I took out of my NAS after replacing it with a seagate exos 18TB.
My question is, to be able to play the games you need first a 2TB Fat32 partition for the game cube set (eur/jap/us) all uncompressed.
Because Nintendont doesn't work wtih NTFS.
And the rest of the drive NTFS for all Wii isos?
I'm looking into NTFS for the WII games to avoiding having to split the files with any backup manager.
Is this the way to go? Can this be done and should work?