Not stupid questions at all! I've heard that it's pretty usual for first-time gardeners to make their gardens too big, get too ambitious, and then get overwhelmed. That's sort of what I did, but it still worked out okay. Things to think about regarding garden size: how much time do you have to garden (are you homeschooling some of the time, how much help will your kids be, how much trouble will your kids be, etc), how much time will your garden take. This second bit depends quite a lot on what you're converting to garden. I don't know what your weeds are in MO, but if you're trying to garden in what is now a patch of weeds, there will be a lot of weeding. There are some weeds that quickly move into newly disturbed soil . . . here it's quack grass, but you don't have that there (well, my parents don't in AR).  Anyway, my point is to look around and see if you think it's going to be a ton of weeding or if it might not be too bad. Since this is your first time, you won't really know, but you'll probably be able to get some idea. If it looks like weeds are already flourishing, scale down a little. Especially with new gardens, weeding seems to be the major time draw.
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Harvest amounts: This really is extremely variable, depending on variety, climate, weather, growing season length, and pure luck. But I'm a data nerd and put together a database with rough estimates of harvest amounts for a bunch of plants (from books, not from my personal experience). It's in metric, so it will be for strange measurements, but I'll convert it . . .
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green beans - about 2-2.5 pounds per three feet of row. I don't get anywhere near this much, probably because our season is short and because I always miss some until they're huge and hard - which makes that plant stop flowering. Probably you get higher yields per plant with less plants, because you can pay more attention to them.
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peas - 3 pounds per three feet of row. This is probably about right for me, if you're counting the shells. I try to grow sugar snaps, because you can eat the shells as well as the peas, so you get a lot more edible stuff out of the same area. Shelling peas freeze better, but about 2/3 of the harvest is shells (which make great compost or animal food, but it's labor intensive). I like peas so much that I grew about 30 feet of sugar snaps last year (and ate nearly all of them fresh), and another 30 feet of shelling peas, which when frozen lasted us until maybe December. I think in your climate, they'll still be producing until November.
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carrots - 7-8 pounds per three feet of row. I don't think I get this much . . . seeding thinly or thinning them out well when they're about 4" tall really helps to increase yields. And big varieties like Royal Chantenay produce a lot more per foot of row than smaller kinds, like the half-long types.
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cucumbers - a different book I have recommends at least three plants per person. I don't know if it's me, or my climate, but I've planted three plants for the last two years, and each year we've gotten about 5 small cucumbers. They need a lot of water, I think.
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peppers - 3-6 plants per person (this is for sweet peppers). I think last year we had 8 plants that produced, and that was just about right for the two of us. With a longer season and better (warmer) growing conditions, you would get more production, but it would also be more spread out . . . and you could pick them when they were actually ripe!
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lettuce - 2 pounds per three feet of row. I don't know about this, but I do know that the way to plant lettuce is spaced out in time. Every year I intend to do this, and fail. I would plant no more than two to four lettuces at a time, but I would do that every week. Just think, how many heads of romaine lettuce would you eat in a week if you had free reign? Plant that many every week throughout the growing season. It's a great idea. I never manage to do it, but I hope you do.
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broccoli - 2 pounds per three feet of row. I have yet to get a broccoli, so I can't tell you how true this is.
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cantaloupe - I don't know, but my parents always end up with a lot of cantaloupe and give a bunch away, so either they plant too many plants or they're very productive
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pumpkins - this really depends on the variety and what you want them for. I grow pie pumpkins, I had three or four plants last year, our season is short and it was a pretty cool rainy season, bad for pumpkins. I still have a few left, some of which are getting moldy and need to be tossed. Pumpkins take up a lot of room, but they're a lot of fun to grow, so I would just grow up to three if you have room.
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dill - this is the easiest herb to grow, at least up here. It will pop up absolutely everywhere from your first year on, if you let it go to seed. They're easy to pull and make weeding smell nice. If you're planning on pickling, or really like dill (like my DH), I would grow about 10 plants. If you just use it once in a while, probably two would be plenty.
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basil and oregano - no idea.
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Finally, I don't know if you don't like it, but okra grows like mad down there - you just give it somewhere to climb and it goes nuts. I think it would be fun to grow, because it is so vigorous, and my parents haven't reported any pest problems with it (unlike tomatoes, for example).
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Have fun!
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