First of all, I don't see a "print button" anywhere. The bottons I see are "Rotate," "Edit," "Slideshow," "Card," "Calendar," "Book," "Email," and a small search box. Of course there's a "print" option in the "file" menu. If I open that dialogue box there's a small window that says "Size" to the left of it, but there's no "custom" option. Just 2x3, 3x5, 4x6, 5x7, and 8x10. That's for "Letter" paper with a "Standard Prints" style. Other options besides "Standard Prints" are: "Contact Sheet," "Full Page," "Greeting Card," "N-up," and "Sampler." None of those options allows a "custom size."
At the top, under the Printer is a box that says "Presets," so I changed that to "Photo," but it didn't change any of the size options.
Well, 5x7 is close enough. I can't probably put a small border around the picture instead of cropping it at the very edge, and when it prints 5x7 the image width will be 6.625 inches.
But it sure is a roundabout way of doing something that ought to be simple and straightforward.
As for the slider bar, what's silly is the overwhelming assumption that the image wanted will be digital. We've left any conventional expectations about photographs so far behind that "image size" is just assumed to not translate to the printed image.
Strictly speaking one probably can't define an image size using the old film technologies either. One would probably have to order 5x7 and just make do. So there's not an actual loss of capability, just sort of a failure to optimize.
It would, of course, probably be very confusing for people to actually have to define the printed image size. I don't know. I'm just guessing.
BTW, I'm using an HP Photosmart C4385, so the driver may not allow customized adjustment. That is, it may be a printer thing. I've had a lot of problems with the HP Printer driver in the past, too. Took me about 6 tries to get it installed so that it doesn't crash the system.
In his video "Time Management" Randy Pausch quotes a guy who says: "Computers are really really fast, but they take too much time." That about captures it.