So, my story takes place in a fictional Idaho town called Serpent's Yard. It's a town built along a strong river coming from an uphill lake and coming to a waterfall at the edge of town. Initially it was settled by Mormons in the mid-1800s, but a few years after it gained momentum as a town, it was the subject of an alleged cryptid sighting when a giant sea serpent allegedly was seen silhouetted in a flash of lightning, bringing her mighty tail down upon the tower of the local Mormon meetinghouse. Since that day, the town gradually became something of a tourist hotspot, most notably on Halloween. The people of the town take Halloween very seriously owing to their heritage of allegedly having a sea monster living in their lake, and have a big parade and a penchant for some really fancy Halloween decorations. As a result, a lot of Halloween fanatics (and a great deal of actual literal monsters, unbeknownst to them) flood into the town every Halloween. It also has a college nearby, though there are no concrete details about how big it is, just that it's definitely associated with the town and called "Serpent's Yard University". I want this to be a fairly small town most of the time, but I'm concerned that some of the above details might make it so that the town would have to be way bigger than I'm imagining. Based on what I've described, what would be a safe minimum population I could say it has most of the year?
A couple of Canadian towns with a lot of tourist traffic are Banff, Alberta, with a population of about 9,000, and Niagara-on-the-Lake, with a population of about 17,000
15K seems like a decent number. If it's something of a tourist spot there could be a few more hotels, BnBs, and cheesy resorts in the area. The latter would probably have cryptid themes like Roswell has alien themed hotels, I'm guessing. If there's rivers and mountains the town would have rafting and climbing activities to attract more tourists. Depending how seasonal the tourism is, they would probably have seasonal workers either from oversees or college style interns. When I lived in North Conway, NH we had thousands of Eastern Europeans come over for the summers and lots of college kids on break. The resorts had housing and shit for the employees. Once you get into the 20K population range, especially in a remote area like Idaho/NH, you need some kind of local industry to support the population, which is totally possible. But the problem we had in NH was there wasn't enough housing to support a larger population. Too many mountains, rivers, and undevelopable land. I'm kind of guessing at those population numbers. I'm sure you can massage them into whatever the story needs.
Based on this description, it sounds like you are looking at a town in the Idaho pan handle. You have 4th of July lake. https://en.wikipedia.org/wiki/Fourth_of_July_Lake and https://en.wikipedia.org/wiki/Custer_County,_Idaho I think these will give you what your looking for.
Depends on just how mega you want the town to go with the Halloween celebrations, but you could go for a town like Lewes in Sussex, England, for comparison. Not that similar to your town in general terms, especially being in the wrong country, but for the Guys Fawkes night events, and that Sussex is a weird county. Lewes has 17,000 people, but every Guys Fawkes night people come from all over the county, country and world for a very elemental feeling night. When I went there was a train strike so 'only' 80,000 people attended. Every shop was boarded up. As crowd members we couldn't move, were crushed between boardings, other members, and burning torches that took one of my eyebrows off and set fire to more than one spectator's coat. There were processions of hundreds of masked members of about a dozen competing firework societies, in their stripey jumpers, pushing burning barrels of tar down tiny narrow streets, lobbing bangers, burning effigies of dead paedophiles and living politicians, throwing lit fireworks at people dressed as the Pope who were trapped on top of burning scaffolds. That was apparently a quiet night. What I'm getting at is that a small town can have a very oversized special night of the year where everything goes totally mad.
Exactly. My first thought was of Sturgis, SD, which has a population of around 7,000 but hosts an enormous motorcycle festival that attracts 500,000 people or more every year. At that point, I assume, the entire town's economy becomes built around that annual boom.
First off, being a Mormon and can tell you, we don't have towers on our meeting houses. The only way we would is if we purchased a previously built church, which we would if it was more cost-effective than building one and the area has a large enough population to warrant it.
I hear Waco, TX does a pretty epic Halloween. To the point it causes problems, but that can be said about pretty much anything involving humans these days.
Roswell has a population over 48,000 and while it attracts "alien hunters", it doesn't have an annual event. It does however, support extensive agriculture/pastoral infrastructure. Amazingly enough, it is home to one of the largest mozzarella factories. It is also home to one of Eastern New Mexico University's campuses; the main campus is about 9 miles northeast.