This has taken me years to figure out how to plan the size of the garden right, I hope this year I get it right.
Here is what I have learned:
1. Do NOT be fooled by the baby plants, they are all thriving monsters that will grow in places you never imagined.
2. When the seed packet says "plant 12-16 inches apart", listen to them.
3. It is okay to thin out the seeds. Typically you may plant three seeds in one spot and then thin out the "weaker ones". The breaks my heart but if it isn't done none of them will survive and then you just have a weak pathetic garden and that isn't any good.
4. DO NOT underestimate vines in the NW. My zucchini last year was 15 feet long, the green beans grew 10 feet off the ground (hard for picking at 5'4'' tall), and my cucumbers were about 5 feet tall. Make sure you have room for them because if you don't they will just grow wherever they want and then you might have a problem on your hands.
5. Tomatoes are huge, I think most people who grow around here can attest to this. Be prepared for them to be at least 3 feet wide and 5-6 feet tall. They require a lot of room and lots of trellising to keep them upright. They will also produce a ton of fruit if the weather is warm so be ready to harvest 2olbs of tomatoes a day once they start coming in. One year I planted 20 tomato plants; I didn't have a plan for them when they started to ripen and at least half of them went right back into the compost bin before they could get used. The next year I planted 5 plants.
6. If you have a small area to garden find things you can rotate based on season and grow plants that you will love to eat. There is nothing better than walking outside and cutting off a bunch of lettuce from your garden to eat with dinner. Grow your garden the way you would want to eat from it.
7. If you have a huge area and want to grow enough to get you through the winter then plan more strategically. Do you buy a can of tomatoes every week? If so, grow 30 tomato plants and plan on canning the majority of them. Economies of scale can be a great way to plan your garden. Shallots are expensive and easy to grow so they might be worth growing to keep for the year but potatoes are cheap and available year round so they might be worth skipping. This really depends on what foods you eat a lot and how easy they are to come by, preferably from another local source.
8. Lastly, do not forget to plant flowers. Zinnias are my favorite because they are easy and they make great bouquets all summer. It is easy to plant some flowers among your vegetables, especially tall plants like sunflowers. They add a happy colorful face to the otherwise green garden.