I don't think you can convert to inches like that. If you use large pixels, say one inch by one inch pixels, that'd be 851 inches by 315 inches. If you use small pixels it could be vanishingly small. My advice is to try a picture, use it, if it's not right, resize it. There should be painting type apps out there that allows you so size a picture to so many pixels by so many pixels for saving, anyway.
Er yes... MS Word for pictures ? Go download a decent painting type app. They allow you to add text and even orient it and position it wherever you like. "Horses for courses", and all that.
If you use GIMP you have everything you need.
You can scale the image (Image / Scale Image) or crop the image (Image / Canvas Size) to any specified pixel size.