A tablet cannot completely replace a PC. However, for people who only need to surf the 'Net, read e-mail, read e-books, and need a few apps, a tablet could be a PC replacement.
The larger the better in most cases, but I like 7" myself. Much better for reading books. Larger tablets are great for magazines and surfing the Internet. It's really just personal preference.
A tablet is a computer. It's neither harder nor easier than a PC. It just depends on the person. I know people who take to tablets like they've been using them since they were born, and I know others who struggle with them. Take your wife to a store that carries several brands, including iPads. Let her play with them and see what she's more comfortable with.