I have a Coby DP-758 Digital picture frame. I cannot change the image size. The pictures do not cover the full screen area. I have looked at the setup and there is no apparent way to change the size or resolution settings.

The best size is 480 x 275.

Hold in the > button on the top of the frame

Most digital picture frames, especially the less expensive ones, do not provide for image scaling or size changing. They do a best-fit with the pictures provided on the memory card/stick. The end result is that some will get streched, distorted and pixelated. You have to prep the pics for them to display cleanly.
The solution is to determine the native resolution of the picture frame in pixels-by-pixels. This should be on the documentation that came with it, or you'll probably find this on the manufacturer's website if you no longer have the docs. Then using an image editing application, scale the pics to suit the resolution of the picture frame.
The best results will be from those images that you scale and crop to exactly match the resolution. Not all your images will match up so easily, so there is are two quick ways to get around having to do so much work on each picture. You'll need an image editing program for this.
Easiest (if your digital picture frame works this way): Usually digital picture frames do a best fit to the image using the pixel dimension of the longest side. If you scale your picture so that it's longest side (vertical sides for "portrait" and horizontal sides for "landscape" pics) matches that of the digital picture frame resolution, and the other side is not larger than the digital picture frames resolution for that side it should display without distortion.
Almost as easy: Using an image editing program that uses layers such as Photoshop or Paint Shop Pro (PSP has a free trial download) you can create a blank black image the same size as the resolution of your digital picture frame.This is your size template, black so that it's not noticed on your digital photo frame. Then take your picture, place/paste it as a new layer over the background template and size/move/scale it until you're happy with the picture. Then save the file as a JPEG. This will "flatten" the file (merge the layers and make it one image for the digital picture frame) and retain the correct resolution. This is also a format tyour digital picture frame likes. These programs use excellent buit-in scaling processes that will give you a clean image that now exactly matches the requirements for your digital picure frame. The picture should now display as you intended it to.
Hope this helps, and please rate my answer.

If all the pictures the frame is displaying are small then you might ask yourself is this the display size that's meant to be. I know that the image is not typically supposed to go all the way to the inside frame edge.
On the other hand if some images are smaller than others, I would say that the picture file images being displayed are just smaller and can be formatted or enlarged by a picture program on your computer quite easily. Image size is what you want to change while maintaining ratio.

You might read up on the software to find out if images are set to automatically stretch to screen size, display correct size, tile.. etc. You've probably already done this.

Let me know if this helps

Go into photo mode and press the right hand button on the top of the frame. That should put you in full screen mode.