the overall point of community gardening is to make plans based on the interests of people who are planning to participate. to say that the garden is for hobbyists is kind of meaningless in a way and tends to suggest a pre-determined point of view about who the gardeners are and what will interest them.
starting from the perspective that you are starting a community garden, holding back any of the design or soil prep until you've got a core group of people. if i remember correctly you have a lot of land available. in this case the core group should be at least 15 people.
starting a garden well takes a little time. do not give in to impatience. the startup phase is the most important for the longterm well being of the garden.
how do you find a core group? lots of outreach, beginning close to the garden land, reaching out as far as you've got energy. outreach=door to door flyers, flyers at community meetings, churches, schools, etc etc. outreach can be time consuming and not feel terribly fun, but its very important.
are you asking people to sign up for a plot? no, to come to a meeting and learn more about the garden. its going to take a series of meetings to explore all the questions you had about size of plot, design, other potential uses of the space (kids plots, wheelchair height plots, fruit growing, herb plots, native plants, donation plots, sitting area, composting area, etc etc), guidelines et al.
what comes of this is that it is a community garden, open first come first served to everyone near by. it will include a variety of incomes and backgrounds and motivations for growing, from relaxation, to making friends, to growing specialty crops, to saving on grocery expenses, etc etc.
you don't want to pre-set these things for other people. except that of course what will happen is that this deciding will take a series of meetings. at the first meeting you will ask some of the people to help with ongoing outreach so that you are continuing to reach out all through the decision-making phase. after the garden breaks ground, new people will be continue to be welcomed until the space is full, and they will participate in annual election of officers as well as annual review of guidelines and modest consideration of design changes that might come along.
there is often a resistance on the part of new community garden planners to the meetings process phase of getting started. no one likes meetings and you are afraid that people won't come, that they won't come 'til they see it get started. have faith, understand that the meetings are necessary, communicate this to the interested gardeners, promise them that the meetings are valuable and that they will fade into the background once the garden has gotten itself started.
judy tiger, washington dc