I have done 5 small festivals this year like the one you are talking about. Have one this weekend that's both Saturday and Sunday actually and it's the last one of the season for me.
The way I like to judge how much to make goes off a lot of what is mentioned above by everyone but I will add in my check list below:
-Projected Attendance (I've found its normally about 80% of what they say and even though rule of thumb is 1/4 per person, I normally do 1/3 a lb per person on meat)
-How late in the festival/ craft show season is it?
-Ask the event staff what previous attendance has been in the past 3 years. Then ask for a food vendors number who has been there before so you can call and ask them (has it gone up, down or stayed the same?)
- How many food vendors and how close are they to you?
-Are there food vendors there also serving BBQ? (sometimes it's a festival no-no to allow more than one but it happens)
-Check the festival map, where is my spot? (is it a good location or did they stick me in a corner?)
Once I weigh all of these points then I try figure on how much to make. Keep the number a little conservative because it's better to fire up the cooker Saturday night to make more than it is bringing home 30 extra lbs of pork in Sunday and not having the freezer space. 80% of the time we have been spot on. Festival goes from 10a-5p and were sold out by 4 and cleaning up as much as possible without breaking everything down to go home. However I will say I have been burned once or twice as well. Rain rolled in on one and the other there were twice as many people that attended than was expected and we sold out 4 hrs before we could leave. Doing festivals like these are a gamble sometimes if it's your first go around and I will also throw out there it's good to keep record of how much you sold and what sold the best. One show you will sell pork like it's the last time people will ever get a chance to eat it and then the next you will sell a side worth of beef.
If you get to the festival and it's not what you expected it to be and there's not as much of a crowd try to find a way to stand out above everyone else if possible. You want (good) attention brought to your set up. A few things that have helped me has been posting on a dry erase board during the slow selling times (2-3 pm) that it's ''Happy Hour!" 1$ off all sandwiches and platters. You may pull in a few extra customers that way. Also I like to try BBQ trivia with people as they walk by. And last but not least a good game of Rock, Paper, Scissors for a $1 off is a oldie but goodie. 2 out of 3 wins (always lose on purpose to help make the sale)
As far as sides, the thing I have found is if you have fresh cut FF on your menu, that's the #1 seller with a platter. Make sure you designate someone to handle just the fryer and pay them well because they're going to work like a rented mule! Potato salad normally is the last thing to sell along with baked beans and Slaw reigns supreme right behind the fries along with small bags of chips.
From the drink perspective I normally get 2 cases of water, 2 flats of Coke, 1 Diet and 1 12 pack of Sprite for a 2 day festival.
I hope this all helps you out and good luck!