TRUCKs come in varying widths and heights. The most common width would be the narrowest width, called 5.0(Venture) or 129(Indy) or 145(Thunder), etc. This width is meant for decks 7.75" and below. However, most people don't bother matching their truck widths to their decks, and will just use the narrowest width for regardless of deck width.

BUT If you really wanna match the widths up then feel free to get a wide truck (139) for about 8" decks or a very wide truck (149) for about 8.25" decks.

Aside from matching widths, wider trucks are also meant to be more stable and are desired by people who skate bowls and vert.

Truck heights usually come in LOW and HIGH, and sometimes MID. The difference between LOW and HIGH is very minute, about 3-4mm.

Generally, HIGH trucks lets you run larger wheels as you have more clearance, and will usually let you turn sharper without wheelbite (wheel rubbing the bottom of the deck).

HIGH trucks will usually have a higher hanger which better protects the kingpin and will let you have more grindable meat before you start grinding the kingpin (which is really annoying).

LOW trucks are thought to be more stable as it makes your board lower to the ground.

Other than those points above, there is a lot being said about how the height of trucks may affect your skating but personally I believe they make no difference whatsover so just use whatever is comfortable for you.