Hi Gilmore Girl,
I think the machine I got would be plenty for doing what you plan to do. What you described is a very similar use case to mine actually. They key is the ram, 8gb. For the VM you want to run, and the software you'll want to run inside it, that config should be sufficient, but 16 may be nice just to be safe.
I would ask you this: are you running Lightroom in OSX or your windows VM? If you're running Lightroom, and Photoshop in the VM, especially at the same time, you might want the full 16gb of ram in the MacBook, so you could devote 8gb just to the Windows VM in that case. But if you plan to run Lightroom in OSX, and only photoshop in your Windows VM then you could easily get away with the 8gb (in fact, I think 8gb would be fine anyway, its more about having some extra just in case).
I am using a big screen at home too. I'm using a 2560x1440 27" monitor, and the machine will be fine for that. In fact, it can run 2 of those at once
😛
Regarding the speed differences, the processors in the 13" are plenty fast for Lightroom and Photoshop. Some big renders with tons of layers might take a little more time than they would on the 15" but not a lot. The awesome form factor of the 13" is way more of a benefit than the slight loss of speed IMO.
One more tip, and I may get in trouble around here for saying it, but there are cheaper high rez screens you can get than the Thunderbolt screen, which are still great. I have this one and it rules:
http://www.amazon.com/gp/product/B00906HNZU/ref=wms_ohs_product?ie=UTF8&psc=1
Feel free to ask me any more questions or clarifications of what I wrote.