We have three paddock areas attached to the barn and the smallest outside area is about 30 ft. x 50 ft. This area is most often used by three of the older minis and they have plenty of room to walk around. I'd say it is a comfortable living area and one pony would certainly have enough room to move and stretch.
From each paddock area, the minis can enter the barn which is also separated into parts by heavy gates allowing for separate feeding areas as needed.
Ok, so the horse will have 2 fenced acres to use part of the time? [If so, I'd make sure he/she gets to spend plenty of time in the bigger area.] I wouldn't go smaller than 25x50' for a drylot, if they will be spending quite a bit of time in the area (that's 1250 square feet, and probably a bit small for a 12H Shetland (it's the approximate size of my 36" mini stallion's run, summer he has a small grazing area in addition to his regular run). [800 square feet might be enough, but bigger is almost always better. 800 is 20x40', so my recommendation is a bit bigger, but that little bit could make a big difference in his happiness.]
Not necessarily , just because your paddocks are smaller doesn't mean you will have an unhappy pony. There is a combination of many things that will make a happy pony. Exercise, good quality feed and love and attention. As I said in your other thread , a pasture mate will help too. There is heaps of other contributing factors to a happy pony, many relate to general care. Dentist, farrier, grooming etc.
Where you live, are there any local clubs around that you could join??. Many first time horse owners gather really valuable info from these clubs and is often a great way to start out.
If only for a portion of the day or at night that's fine. Remember that most things are not a 'rule'. You can make a lot of different situations work depending on how you manage it. I keep a 13.3 pony and a miniature in a 32 x 80 dry lot majority of the time. They get turnout, exercise ect and are completely content. I only have a 1/2 acre paddock for them for grass and running but its enough they can get a good gallop in.
Also think lots of horses are stalled in 12x10 stalls daily, though I don't recommend that, every situation can work if you put the effort in. Your biggest thing will be picking manure to manage mud in the area.