I get asked fairly often by customers how to pick the right size trucks for their deck. There are a few things to consider, but it is in essence pretty straight forward.
This is the most important thing to keep in mind when determining the right size trucks for your skateboard.
In general you will want your axle to be close to the same width as your deck. So on a 7.25" wide deck, you will want an axle around the 7" to 8" mark for your axle, or around a 5" hanger. Similarly on an 8" deck, you will want something around the 7.75" to 8.5" mark for your truck axle, somewhere around a 5.25" hanger.
Remember though, the hanger width is not the same width as the axle, it is actually shorter.
The hanger is the thick section that the axle runs through as you can see in the photo. The axle then extends out from the hanger.
Trucks come in 3 main height profiles - low, standard and high. You should keep this in mind when picking a set of trucks because it will influence the size of the wheels you can use on your deck, and subsequently the type of skating your board will be best suited to.
Low profile skateboard trucks are favoured for tricks as they offer greater stability by lowering your centre of gravity on the board. This gives you a more stable platform to do kicks and flips from. You do need to use smaller wheels though. In general, you want to make sure they are no bigger than 54-55mm.
Standard profile skateboard trucks are good for all round usage, tricks to cruising. If you aren't really sure what you want to do with your skateboard, these are a good choice.
High profile skateboard trucks offer a better platform for cruising and carving on because they can handle bigger wheels, which can get you more speed. Typically you will be looking at anything above 55mm on higher profile trucks.
The other thing to consider is that if you have your heart set on some low profile trucks but want to use bigger wheels, you can use risers to increase the distance between your deck and axle.
Wheel bite is when the wheels hit the bottom of the board when you turn. This can happen if your kingpin is too loose or if your wheels are too big for your trucks/your trucks are too low for your wheels.
Trucks generally come with the kingpin tightened to a reasonable level for use with the appropriate size wheels for their profile, so if you stick to the suggested wheel sizes above, your kingpin should be set fine, though you may want to make some adjustments to your taste.
As long as you don't get wheels to big for the profile of your trucks, you shouldn't have a problem with wheel bite. If you think you might, try standing on your board and tilting from side to side as far as you can, like you would in a turn. If the wheels touch your deck than you will need to get smaller wheels, higher profile trucks, or some risers (this is probably your cheapest option).