I'm not sure if you meant 800 x 700 or 700 x 800 as in the first dimension the width is 2/3 the height. If you keep the proportions constrained you can get:

eg.
467 x 700
533 x 800
700 x 1050
800 x 1200

If you don't keep the proportions constrained, you will have a seriously distorted image.

Any time you increase or decrease the image size it will blur. You can use Filter - Sharpen - Unsharp Mask. In the Unsharp Mask window you will have to play around with the settings. Most of my images I use:

Amount: 50-75%
Radius: .9
Threshold: 1