Medium to large. I don't particularly care for churches with less than 75 people, too much care, maintenance, and work ends up on the shoulders of the congregation. I also find children's programs in very small churches to be sorely lacking or non-existent.
Usually if you have a suggestion for something the response is "that is a great idea, we don't have the resources for it, but if you want to spearhead it....".
I agree that a church wants to be large enough that things can actually be done, although that's about willingness to get involved rather than sheer number of people alone. Our church has a lot of older people in it. Some of them are still active in the church, although a disappointing number take the attitude that they've "done their time" and now they want to let the younger ones do the work. Except in a lot of cases, relatively speaking, they are still "younger ones", and the people they think of as "younger ones" are already maxed out and burning out.
Once a church gets past a certain size it becomes too easy to get lost in the sea of faces. Personally I think if a church is big enough that individuals can get lost in the sea of faces it isn't really much of a church any more.
Like cities, there are advantages and disadvantages to small and large.
But I prefer a small family church. They tend to be closer and more "family-like" and there are more opportunities for involvement. I also like how the pastor-people relationship tends to be much closer and more personal.
Having been a part of large churches, and one that is technically verging on "mega" I have found that if one desires to be active and reach out to the pastor and leaders one can be as close to the "inside group" as you want, but one must take the initiative to be active. Compared to a small church where you must be active to keep the church going a larger church requires some effort because one can be passive with little effect to the larger church body.
I like smaller ones because there is a real sense of community there. In the larger ones, such as I used to know, there is a certain mechanical and impersonal mood. I often left worship, in the large church I once attended, feeling like I'd done my duty but that's about all.