I have been working on Windows Store and image related project recently. Check my previous bid ask for reference from the person there.
I have working on Windows Store application for 2 years and I see that you want the app to be in Kiosk mode where only one metro app is started. As Windows Store app already have access to the printer and it will be easy to do the printing. We will focusing on developing the app and the backend for you.
I will use Facebook Graph API for accessing the user data such as uploading the photo as you said.
* I must be able to print on various sizes (not being photo sizes) For example, I want to print on 105 x 72mm (this NOT a standard photo size. So there is NO setting in printer driver for this size, but I know a work around for this so don't worry about this point)
This point, i just tested to print and the printer pane only show list or pre defined paper size. Because the main intent is the photo, we will only manipulates the photo size rather than the paper size which is not available for change. If you say you have workaround, im open for it.
I will use git hosted in bitbucket so you will know my progress everytime and you can monitor my progress each day.
Contact me and discuss this project further, there are some questions that I have to ask.
Thanks