You need to find a way to make an ISO.

Maybe try AnyToISO or the method below.

1) Go to Finder > Utilities > Disk Utility (or Spotlight > Disk Utility)

2) File > New > Disk Image from Folder

3) Select The Windows 8.1 Pro Folder and click "Image".

4) In Image Format choose "DVD/CD Master" and in Encryption choose "none". Then click "Save". (Save it on desktop).

5) Next go to Terminal, Finder > Utilities > Terminal (Spotlight > Terminal).

6) Use the code below



Change sample.iso and sample.cdr to your target file name. Then enter.

7) Now, try opening Boot Camp Assistant with your USB Flash Drive plugged in and try installing. Hope that works.

Try using the tool in the link below

http://go.microsoft.com/?linkid=9815935

Instructions on how to use it

http://support.microsoft.com/kb/2809219

You won't be running OS X and Windows 8.1 at the same time, right? So 4GB is enough (depending on what type of work you'll be doing).
I think MBA is a better choice due to that 256GB of space.

Now if you could upgrade that rMBP to 8GB+256GB, that would be perfect . Anyway, if you are serious about using other OS on your Mac, get at least 256GB of storage space. Using 128GB SSD is possible, but you'll have a hard time managing it.