According to their help
1. Open Picasa.
2. Select your image and press the “Export” button.
3. Under "Image Size Options" select the "Resize To" button and adjust the slider (e.g., to 700 or 800 or whatever). Please note that the number of pixels you select with this slider determines the length or height of your photo (whichever is longer). The other dimension will be determined automatically in order to maintain the aspect ratio of the photo.
4. Select the desired image quality for your photo using the "Image Quality" drop-down menu:
- "Automatic" will preserve the original image quality
- "Normal" will balance quality and size
- "Maximum" will preserve fine detail for large file sizes
- "Minumum" will yield some quality loss for small file sizes
- "Custom" will enable you to select your own value