A good guide for size is a 2" long by 1/2" wide and 1/2" tall brick. You don't really need to get a ruler out or anything, just go with something that seems about the right size to eat. Too big, and you'll have a hard time picking it up. Too small, and you'll have to eat about 30 pieces. If you clump the rice where your fingers meet your palm, and curl your fingers to square off the brick, it will give you approximately the right shape. It's hard to describe exactly how it's done, and I don't have any pictures of that step, so you'll have to experiment with what works for you. Compression of the rice is important, or it will fall apart when you try to eat. Once you have a brick ready, top it with a slice of fish that is just a little bit oversized, so that it drapes slightly over the edges. Squeeze the fish and the rice together, so it won't fall off, and you're done. It may not look like what you see in a restaurant, but they've got more practice in than you. On the imitation crab rolls, I took a small strip of seaweed and wrapped it around the roll, to make it look awesome and to ensure the topping stays on.

So, now that you've got making nigiri down, let's try some rolls, or norimake. These are what you see being served most often. It's a roll of fish or vegetables with a wrapping of seaweed (nori) on the outside. They're relatively easy to prepare, but take a lot of time compared to nigiri.

For a normal-sized roll, you'll need to put a full sheet of seaweed out on a bamboo rolling mat. Protip: turn on a burner on your stove, and drag the seaweed across it a few times, to make it easier to work with and less likely to split. Spread rice thinly across the seaweed with your hands (remember to wet them down often), making sure not to leave gaps in the middle, but do not pack the rice down, rolling it will take care of that for you. You'll want to put a line of whatever is to be in the roll down the bottom side of the seaweed, leaving enough room to start the roll. It will look something like this:
  
 
 Step 5:

Pictured here is a California roll, minus the fish, with some rolls of fish eggs to the side. I didn't have any tuna left over from the nigiri, so we just used cucumber and avocado. Really, you can add almost anything to these rolls, so long as you slice it up to fit on the seaweed. To begin the roll, grab the edge closest to you, and curl it over the bulk of the ingredients. Once you've got it over, you can start using the mat to roll, like this: