You are going to make mistakes and do things you'd later wish you could change. That's just the way it is. If you do lots of research beforehand, there will be fewer regrets. My mistake list is at
http://www.fishpondinfo.com/problem.htmBut really, even with some imperfections, there really is nothing like looking at a mature watergarden and all the literally thousands of animals and plants that are enjoying and using it. If I had time, I could watch all day. In the heat of the summer when all natural ponds are shriveled up around here, it's most interesting to watch the various insects come in for water. If my pond weren't there, where would they go?
Your space is limited. If you put in a 200 gallon pond, you'll enjoy it and probably wish you'd had the 2000 gallon. There's no reason though that you can't have both! Try out a smaller pond and then later put in a larger one. It can be even smaller than 200 gallons (but don't expect to keep larger fish like koi in that). I have a 1800 and a 153 gallon pond. I have more land though. If money, time, etc. aren't limiting you, then I say go for the bigger pond!
Take your time. Enjoy the process. Getting there is part of the fun!
KoiKrazy lives up to her name. Don't let her scare you off ponding. Most of us only have a pond nightmare once a month. :D