Resize Photos
When you resize a photo in Picasa, you're creating a new copy with an adjusted image resolution (pixel count) and with all photo edits applied. The original photo on your hard drive is never altered.
You can reduce picture size in Picasa by exporting.
Resize by exporting
Exporting lets you change photo size while controlling the JPEG compression (image quality) introduced by your applied photo edits. The result is newly resized copies of your photos, saved to your hard drive. During the export process (also explained in the Picasa Help Center page) you can adjust both the "Image Size Options" and the "Image Quality" settings in the "Export to Folder" screen.
Selecting an image or images
Click the "Export" button
If you don't see the Export button, see https://sites.google.com/site/picasaresources/Home/Picasa-FAQ/picasa/how-to/how-to-install-customized-buttons-in-picasa3
In "Name of exported folder," you can leave the default name or type something such as the word "smaller" or "export"
When typing "smaller" or "export" you can more easily see later where the exported photos ended up. Using the default name might be confusing later.
In the "Export to folder" window, click "Resize to" then either type in a pixel number or adjust the size slider.
The number of pixels you select determines the length or height of your photo (whichever is longer).
The other dimension is determined automatically to maintain the aspect ratio of the photo.
Leave "image quality" at Automatic (see below for more options)
Click "Export"
Go to File Explorer (in Windows) to your default Downloads folder and look for the "smaller" folder
Open each photo and be sure the quality and size is to your liking
Note that now you will have the original larger photos on your computer as well as the resized photos.
It's up to you if you want to keep the original larger photos
Other "Image Quality" options are in the drop-down menu. Test each one to see what works best for you:
Automatic: Preserves the original image quality
Normal: Balances quality and size
Maximum: Preserves fine detail for large file sizes
Minimum: Yields some quality loss for small file sizes
Custom: Lets you to select your own value
During export, you can also choose whether to add a watermark:
Check-mark the "Watermark" box
Type the text you wish to appear, such as: ©yourname