Douvidi
If I'm reading you right then Jim is correct, what you are doing is creating the entire image size including the border you want in PS and then sending it to the printer, the canvas size you are creating in PS is 17"x21" with the image centred at 14"x18". If you are doing that then you would require the printer to be able to print full bleed on canvas which it can't do so it's reducing the size slightly which is why you are ending up with a smaller image, it doesn't know that the border you are creating is white, it just knows that the image size is 17"x21" which it knows it can't print because it would need to be full bleed.
As Jim says, the only thing you should be doing in PS is resizing the physical image to 14"x18" then sending just that to the printer, forget making the canvas size in PS to equal the physical canvas size in the machine.
Once you have the image at the correct 14"x18", in the print dialog box, tell the printer that the media is 17"x21" and that you want the image centred within that space, it will then print the image at the correct size within the 17"x21" media size.
Mat