I'd let Windows requirements dictate the size of SSD that you choose. I don't have enough experience of that setup to offer you sensible advice. TBH, I'm not a huge fan of SSDs for normal use (or at least my normal use); I'm more in favour of plenty of RAM. Most modern OSs will make use of available RAM to cache disk data so the only performance hit is generally the first time you access a particular program or file. Certainly Linux is very efficient in this respect.
SSDs certainly give faster boot-up times, as everything is being read for the first time, but that don't really matter to me. Most of the time I hibernate my computer, and it comes back up plenty fast enough for me.
Whatever you choose I would still say that a 32GB virtual drive should be plenty for a Ubuntu VM; RAM, no of processors, video RAM, etc. is a matter of choice, but all of these are easily changed so you can tune the VM to your particular requirements. I use a Fedora VM running on OS X for my development work; I've given it a 16GB virtual drive, 2GB of RAM, 12MB of video RAM, and two processors (I have a 60% execution cap on those processors, which means they never use more than 60% of available CPU on just two of my eight cores - four real cores + hyper threading). This works just fine using Eclipse, GCC, and various emulators running within the VM (VMs running on VMs - sounds crazy, but it works for my particular needs - home brew OS development), but I'm probably going to have to up the virtual disk space next time I update Fedora to a new release (real soon now).