How do I make all the pages the same size, say 8.5×11

Try to use

Tools | Resize

or

Tools | Canvas size

Resize vs Canvas Size

What’s the difference between the Resizing and Canvas Size functions?

“Resize” command allows you to resize the entire image.

“Canvas Size” command allows you to resize the canvas only (background), without resizing the image.

Available programs:

• Advanced TIFF Editor Plus (free download, home page);
• Advanced TIFF Editor (free download, home page);