Hi Jon,
I did not post the original answer to resizing on Picasa, but.....................
At the bottom of the hoto that you are looking at there are a row of tabs
'Share' 'eMail' 'Print' and 'Export'
click on the Export tab and a new window opens and among the options is one for resizing- chose your preference and then any other option that you need, click on Export, a new window opens with a highlighted file (the same name as your main file that has the photo in that you are exporting) double click on it and a new file opens with your exported photo in, close that file by clicking on the red box in the top right hand corner, that file then clears and you are back on the main file to chose your next photo. A new section is then automatically added at the bottom of the folders list on the left hand side of the library screen.
I hope that this helps.
cheers
Mike