Make sure underground farms are muddied. Emptying a small pond works well, and you can seal it up and let it refill with rainwater afterwards. Planting in the caverns works well too. Enable the crop you want for each season, and remember the view defaults to the current season when selecting farms.
For size, I've been doing 1x1 farms, I've found that a tiny, ~40 tile total farmland (above and below ground combined) with skilled farmers supports over 100 dwarfs. I keep farming enabled on a bunch of folks to start until I have some high-skill farmers, then I remove the labor on everyone who still sucks at farming. I leave "everyone harvests" enabled though, and periodically check farmer skills and enable the ones who have been gaining skill through harvesting. It's also a good way to see who is idle, if they have free time to harvest they obviously aren't doing anything else.

As your farmers gain skill, the batches of plants harvested get larger, but it's planter skill not harvester skill that causes this. You will find that your farmlands "grow with you" as your planters get more skilled, but the more planters you have, the more the skill gains get spread around. In other words, 10 farmers will be legendary in short order, but 100 farmers will not. For a high-population fort with low skill farmers you'll find that you need 100 farmers and 1,000 tiles worth of crops to keep up with the demand, instead of 10 farmers and 100 tiles of crops.
I don't fertilize but that doesn't mean it's a bad option, only that I personally don't need it. Fertilizing can provide a huge boost to crop production though, with minimal work needed and no additional land.
Fallow: like others said leaving a farmland fallow doesn't provide any bonuses or anything, it only leaves that farmland empty for that season. It may be useful to temporarily deactivate a field for a crop that you are overproducing.
Remember you can send herbalists out to gather plants, if you find yourself running unexpectedly short on crops. It may be a good idea to gather plants occasionally even when you don't need them, to train your herbalists up for the time when you DO need them.
.. sorry, I'm apparently channeling Verbose Man.