You really have to shop around if you want to win the jackpot. The size of the jackpot changes dramatically depending whether or not you play at the free sites or the pay to play games. Let's start with the free sites.
Most free sites offer progressive games so the jackpot amount grows until someone wins.
Some games grow by 50 cents a game and others by a few dollars. Most sites tell you how much money you can win and by how much the prize grows in the how to play section. Also check the list of recent winners to see how much they won. There's progressive coverall with new games starting every 15 minutes. It's harder to get jackpot info at the pay to play sites.