If you don't want to go out of your way for something different, the bar or Tempura Kiku in the Japan pavillion is the perfect place to stop - particularly if you haven't had sushi before and just want to try it - you can order a snack mid-afternoon as you stroll around WS - see if you like it. If you don't, something more familiar will be close. I've heard Kimono's is wonderful as is the CG.
I find about six peices makes a good light meal - depending on the size of the peices. I perfer shashimi (raw fish over rice) over maki (the seaweed rolls). Tuna and salmon are both mild tasting. And the eel is fantastic (and cooked - don't think slimy eel).
Wasabi is hot. The way to eat it is to let a little dissolve in your soy sauce and then dip the sushi in the soy sauce. But if you like the burn, you can smear it directly on the sushi. (If you REALLY like the burn, just eat some wasabi).