If you need to enlarge a photo, you can use Picasa'a Collage button, but understand that you can't really enlarge a photo and get any more detail from it. You will only end up with a larger, but blurrier, version of the photo.
In Picasa select a photo
Click the Collage button. If the Collage button is not visible, see Customized buttons in Picasa
In the collage dialog, select the "Picture Pile" collage.
Move the photo to the center of the collage and grab the little circle "handle" to adjust the size and orientation of the photo.
Change the Page Format to a larger size, if desired
Click "Create Collage" then you can edit and crop
Your result will be a new JPEG that has more pixels than the original photo, but do not expect much in the way of clarity or sharpness.
Use Picasa to resize photos to a smaller size using the Export button. Click here for directions. Resizing using Export does not let you enlarge a photo.