You've got something like a Silver Smoker or Brinkmann Smokin Pit Pro then?
Now's the time to do all the mods, if you haven't already done them. A manifold between the firebox and cook chamber, extended flue, charcoal basket, big drip pan, water pan, wireless pit thermometer, etc. A little cooker like yours benefits hugely from these little and mostly inexpensive mods. Makes all the difference in the world with a big project like a pig.
You can do a small pig. racer style in it. Max size for a not too inconvenient good job is right around 35 pounds (dressed) -- which will feed roughly 15 if the pig is the centerpiece of the event. You won't get more than 20 people unless there's some other meat being served on.
"Racer style" means, head-on, cavity trussed, and the pig laying on the cooking grate with its legs tucked beneath it. It's a really nice presentation. Anyway, your smoker is a litle too narrow for a butterflied pig, and a little too short for semi-racer (rear legs pulled out).
If your little pit is well tuned (that is the temperature levels are fairly even from one side to the other) the pig won't need to be turned. Otherwise it will need to be turned at least once during cooking.
Estimating the time for a whole animal is a little tricky if you've never done it before. You base it on the size of the thickest part -- which is going to be the hams and shoulders. These are actually pretty small on a thirty five pound pig -- you're probably looking at a 4 to 6 hour cook, or as much as 8 if you go very low and slow.
Some people find it helpful to wrap the pig in light wire fencing mesh to help turn it around during the cook, and to get it out of the pit afterwards. But since you're not butterflying or spitting, you don't really need to do it on a pig that small.
One thing a lot of people don't consider is the pig board. You'll need something long enough and wide enough to serve the whole pig on. After he's cooked, you slided him onto the board, carry him to the table, and peel the skin off. If you've cooked the pig hot, the skin is wonderful. But if the cook was slow, it may be too leathery to eat.
Many people don't, but I inject. Your choice can depend on a lot of things but apple juice and white grape juice, mixed 50/50 then seasoned with the usual suspects has won a lot of comps. For years, that was the base Chris Lilly used. Personally I prefer something more citrusy or peachy.
Here's another thing many people negelct. Gloves. Don't forget at least a couple of pairs of gloves that will allow you to handle the pig while it's still hot.
A thirty five pounder can be easily handled by two people. Make sure you have at least one reliable friend on hand designated to help. Best not to rely on your spouse -- at crunch time, she'll have plenty of her own things to do.
That's a few of the basics ... please ask whatever comes to mind.