The biggest laptop I've ever seen was 20 inches. In fact you couldn't actually use it as a laptop! Most people who buy laptops, they never take them anywhere, they just sit on their desk. Laptops are more expensive, they're not as upgradeable, an they get a lot hotter inside so they don't last as long as a desktop.
17 inches is usually the biggest size they have. And yeah, they don't all have the very latest processors in them. On online sites like Woot and Groupon you often see laptops that have old dual-core processors, or even Pentiums! Very cheap too. I'm guessing they're leftovers from years ago.
But you might do better to buy an older desktop, maybe even a used one, and get a big screen for it. A 25" screen today is like $100 new. Look on your local Craigslist and see what you can find in a Core 2 Duo processor, or an AMD Athlon, with 8 gigs of memory. That's a pretty good computer even by today's standards.