For me, I have:
FM5: 40.4 GB
FH2: 42.5 GB
FM6: 52.4 GB
Seeing as they have an upward trend in size, I'd guess that FH3 will be around 55 GB total, perhaps 50 GB day one. I do have all the DLC for the games as well, save for the second FnF pack for FH2.
If you're looking for an External HDD, I got one recently, and it's pretty good so far. I have a combined total of 31 games on my internal HDD and external HDD, and while the internal is entirely used up, I still have 1.2 TB out of 2 TB free space on the external. I have a
2TB Western Digital My Passport Ultra. Really good to me so far, very easy to adapt to the Xbox. Only mild complaint I have is the constantly blinking white light when my Xbox is off. On the bright side, it lets me know it still works!
And talking about large game sizes.....ha...don't buy Halo 5 then! 81.2 GB!
One of "The 50"