#2 - how can I figure out the size of my library so I buy the correct sized external HD?
Go to your Pictures Folder and find the iPhoto Library there. Use the Finder's File -> Get Info command to get the size of the Library. When you purchase an extenral disk, make sure to format it Mac OS Extended (Journaled). Also, allow room to grow.
#3 - what is the best practice for moving iPhoto to an external HD? Do people move over only 'older' photos or just run iPhoto off the external HD completely?
There isn't a 'best practise' there's only what you need to do. IF the Library is on the external, then you can only view it with the external attached. Are you on a Laptop? Desktop? If you're on a desktop it probably doesn't matter. If you're on a Laptop it might. Regards TD
However, after using the iPhoto Library Upgrader, Apple’s recommended path for converting iPhoto 7 and earlier libraries to a newer format that iPhoto 8 and 9 can use, “The result is photos a. Take advantage of the Files app in iOS 13 or iPadOS. The first, and easiest, way for you to view your Photo’s metadata is with the Files app. What you will need to do is open the Photos app on your iPhone. Locate the photo that you want to view the data for, and tap the Share button in the bottom left-hand corner.
Photo File Size Changer
When you export a picture from iPhoto to a file on your hard drive, you can save the picture in its original file format with no loss in quality (and no compression); in another file format such as JPEG, TIFF, or PNG; or with adjustments you made to the picture. iLife can also save images and photos in full, large, medium, or small sizes or even a custom size.
Photo File Size And Printing
To export pictures to files on your hard drive, follow these steps:
Photo File Size Converter
Browse your library and select one or more thumbnails of pictures to export.
Choose File→Export.
Click the File Export tab (the leftmost tab).
Choose the appropriate file format from the Kind pop-up menu.
The Kind pop-up menu lets you pick from these formats:
*Original: The original format for the photo (used by the digital camera — typically JPEG but can be another format), before any improvements were made.
*Current: The original format for the photo after any improvements were made (such as cropping or color adjustments).
*JPEG: Joint Photographic Experts Group (JPEG), the standard image format for Web pages. Use this choice to ensure that the photo is in a standard version of JPEG (if your digital camera’s Original format is specialized).
*TIFF: Tagged Image File Format, the standard format for desktop publishing software.
*PNG: Portable Network Graphics (PNG-24) format, a lossless alternative to JPEG.
If you select JPEG, choose its quality from the JPEG Quality pop-up menu.
Include titles, keywords, and location information.
Specify the image size.
Specify how to create the filename.
The File Name pop-up menu lets you choose from these options:
*Use Title: Use the picture’s title.
*Use Filename: Enter a filename when saving.
*Sequential (and then enter the prefix to use before the number): Specify a sequentially numbered filename with a text prefix.
*Album Name with Number: Use the photo album name with a sequentially numbered suffix.
Click the Export button and choose a folder for saving the file.