Loading the Vehicle:
When you get to the loading zone you will be asked how many are in your group. Then the Cast Member will use that information to determine where to put you for loading. In most cases you will be sent to a specific row. The rows will have an electronic gate keeping them closed. When the gate opens you go through and enter the ride vehicle.
There are a few rides that don't stop when you get on and off. At Disneyland Buzz Lightyear Astro Blasters is one of those attractions. It can be stopped for the case of loading wheelchairs but for the general public you just walk along a treadmill to get on your vehicle. Haunted Mansion is another that you just walk on the treadmill to enter the Doom Buggy. At Disney's California Adventure you will find that Grizzly River Run and Mulholland Madness are continuously moving loading areas as well.
For rides that use a seat belt, you will usually find a yellow strap on the belt near the end you slide into the buckle. You will need to put your arms up and pull on that yellow strap when directed by the Cast Member. This is a safety precaution to make sure you are securely fastened into your seat.
Seating on the Vehicle:
Each attraction at the Disneyland Resort uses a different type of ride vehicle. Some are boat rides with bench seats. Others are formed seats for each person, and still others have a bench type seat that is smaller than the boats. This information is here so that you can find out how many people will fit onto each type of seat.
The bench type seating, seen on Pirates of the Caribbean and 'it's a small world' can fit up to five people, depending on the size of the guest. For average sized people with a couple of children it will easily accommodate a full five people.
Preformed seating is found on Jumpin' Jellyfish, Soarin' over California, and most roller coasters and tends to be on the bigger side. If you are the average size you will have no problems fitting. If you are a bit larger than average it may be a bit of a squeeze but you will still fit in the seats.
Smaller bench seats can be found in almost every other attraction not mentioned above. These seats can easily hold two average sized adults and a child. With two adults and two children it is usually better to split into two groups of two as most ride vehicles can't hold that many people together. In the Haunted Mansion, the Doom Buggies can hold two or three, but not four people. The same is true for most Fantasyland attractions as well.
Next you have the 'car' type vehicle. The Honey Pots for Winnie the Pooh, the Caterpillar for Alice in Wonderland, and Dumbo the Flying Elephant. These are set up to handle two people per row, or in Dumbo's case, two people per elephant if you are an adult and child. Two adults would need to use two elephants. This means that aside from Dumbo, a family of four can all ride together. For the others you can put two in the front row and two in the back row, or one in front and two in back.
Finally, there is actually a car ride vehicle. Autopia is one of the perennial favorites in the younger age group, and even older kids enjoy this attraction. Those vehicles are not made to fit two adults, or two adults and a child. You will need to split up on this attraction.
For the Larger Sized Person:
If you are bigger than average then some of the seats may provide a hardship for you, but most will give you no problems. Currently I happen to be a person in this category so will be able to give you good information on what works and what doesn't. For our last trip to Disneyland I was 100 pounds over weight due to medical issues. Much larger than average. If you wear size 20 - 30 this information can be helpful for you.
The first thing you will have difficulty with are the turnstiles you must go through for most attractions. Due to wide hips I must go through sideways. The turnstiles are typically low, so a taller person may be able to just tiptoe through. On some attractions you can skip the turnstiles so be sure to ask if you are unable to go through.
My rule of thumb has been that if I can fit in the airplane seat I can fit on the ride, but knowing that not everyone flies I will try to give help for you as well.
Some that provide difficult for my size are the 'obvious' rides for children. Those in A Bug's Land do require some squeezing. Jumpin' Jellyfish has very small preformed seats since it is essentially a ride for the younger set.
If you are single riding some of the big kid rides Soarin' is okay, Grizzly River Run works well and Splash Mountain can accommodate large people very easily.
Like Jumpin' Jellyfish you will find smaller seats on Mulholland Madness (Goofy's Sky School when the change is complete). To be honest, Mulholland Madness is tough for any adult. It is low, so getting in and out is difficult and the seating area is positively tiny.
Haunted Mansion uses good sized Doom Buggies but as we got larger and our son grew bigger we have had to split up into groups of two and one. We also split up on the Fantasyland attractions Peter Pan and Mr. Toad.
The bench seats for the 'boat' rides will accommodate less when larger people are involved. In this case those seats that can hold five will only hold two or three people.
While splitting up sounds horrible, please keep in mind that if you split up, at least you still get to enjoy the attraction. The Disneyland Cast Members are wonderful and will usually let you know if it is better to split up without making you feel bad.
A few tips as well:
If the attraction has a seat belt, pull the belt ALL the way out then buckle it. The belt will retract any that you don't need.
For lap bars, sit alone or with someone sized similar to you or the other person will have too much wiggle room - I am thinking specifically of Big Thunder Mountain Railroad here, which is not a ride you can take a toddler on anyway. In Haunted Mansion the lap bar is not designed to be tight so that attraction is fine to have a smaller size person with you.
When you are in a low seat and must get up after the ride ends, use the back of the seat in front of you for leverage, and when you are up, stand on the seat you just vacated so that you can make the step to get out. This is an accepted practice even for those of a much smaller size.