Short answer, you cant. That is, if you have a big family or your fiancé has a big family and you both have lots of close friends, then odds are youre just going to have to pony up and go for the big wedding. But big families and big friend groups dont automatically mean big weddings, so long answer, it is possible.
Consider the Location
Maybe you each have twenty, first cousins spread out all over the country. How likely are they to travel? Did your cousin Becky from California fly out last year for your brothers wedding? If she did, she would probably do the same for you. If she didnt its probably safe to assume that the distance is a little much for the wedding of a cousin she sees every couple of holidays.
Consider the Relationship
Youve both got all those great college friends, Im sure. So many memories with lots of great people. Its natural to think you should invite all your old chums, but do you still keep in touch? Are you likely to still be in touch in five years? It might make you a little wistful to admit you arent as close with certain people, but the ultimate goal, above keeping the guest list from being unwieldy, is to have people at your wedding that you really want there. Trust me, guests can tell when they are an afterthought or an obligation and its not the most fun way to spend an evening.
Do You Both Know Them?
This is an important question to ask, and it will rid your list of a lot of casual acquaintances or those you have lost touch with. Its not a blanket rule because of course there are some people you will both be meeting for the first time at the wedding like relatives or far-away friends. Its a rule more designed to determine if the guy your fiancé sometimes gets a beer with after work, but who has never met you, should be invited.
Be Generous With Plus Ones
I know, this is counter-productive to the small guest list goal, but whenever possible, make sure your guest is able to bring the person they want. If you cant manage to make room for their plus one, then that means you dont really have room for them. Thats because we know people dont want to go to a wedding alone. Its just not a lot of fun. If you know they will know a lot of other guests than its a little different, but use your best judgment.
Keep in mind that all of these suggestions come with stipulations and mitigating circumstances and ultimately the decision belongs to you and your fiancé. But dont stress over the guest-list too much. We all know we cant be invited to every wedding. We all know how expensive weddings are and what kind of seating limitations there often are. We promise we will understand if we cant be invited, and we will still wish you well. If your ceremony needs to be especially small you can rely on your immediate family to spread the word to other relatives. They just had to keep things very small, your mother will report, I barely got invited. And just like that your diplomatic mother saves the day.