What small size did you want?

My assumption is that it is most convenient to have a standalone USB rive with room for FreeDOS, Fractint, and any fractals you create. This eliminates any need for FreeDos to access anything else on the computer. I dumped a huge fractint working directory I have will all kinds of unnecessary stuff, and still have way more than half of 8 gb free. I will give a more surgical answer, but I am guessing that multiple hundreds of megabytes (say 512 mb) is more than big enough. I even have some 32 mb flash drives that came out of new digital camera when they were replaced by large drives, this is probably enough. But why scrimp - the sandisk cruzer fit 8 gb costs about $8. (I only mention this particular one because I have it and know it works). By the way, much larger usb drives probably work, fat32 formatted partitions can be in the terabytes. I had forgotten that. 

Geeks can optimize the heck out of this. For example you could have FAT32 partitions on hard drives for storing fractals. But  I am looked for something turnkey that works most of the time with minimal fuss and special exptertise.

I should also say that I have been a big fan of DOSBox, because it runs fractint really well, great VESA video support, and runs under Windows, Mac, or Linux. But running benchmarks on the same machine shows that Fractint under FreeDos is more than twice as fast as running under DOSBox. I haven't verified that the intentional slowdowns of DOSBox  to make games work right have been turned off in my setup (I think so). In any case, it stands to reason that running FreeDOS directly would beat a virtual machine for speed. I love the idea of using an old machine with no hard drives to generate fractals 24/7!

Tim