Large, high-quality photographs are great to work with when you're making edits but can be obnoxious to share. At first glance, Picasa lacks a function to resize an image. The function does exist, but only appears when exporting images. You can also configure Picasa to automatically resize photos when emailing them and choose from a handful of standard sizes when uploading them to a Web album.
Click "Export" to open the "Export to Folder" dialog window. Ensure the "Resize To" radio button is selected -- this enables a slider you can use to choose a size for your image. The slider provides quick access to common image sizes, varying from 320 to 1600 pixels; if you need your image to be a specific size, type a number into the text field on the left of the slider instead. Whichever way you use to specify the image size, the number you choose will be used as the size of your photo's longest dimension, whether it is the height or the width. The other dimension automatically will be resized to match, maintaining the photo's original aspect ratio.
When uploading to a Web album, you can choose between leaving a photo its original size or resizing its largest dimension to 800, 1024 or 1600 pixels. Photos with the largest dimension set to 800 pixels or less do not count against your storage limit, while larger ones do.
Click "Tools," select "Options" and then click the "Email" tab. Adjust the slider to choose how much Picasa should resize your photos when emailing several of them at a time; the slider provides various sizes, varying from 160 to 1600 pixels. You can also choose whether Picasa should resize photos you email individually to match the size you selected with the slider or leave them at their original size.