Having a trout pond myself, smaller is actually better. Why? Carrying capacity is more dependent upon flow rate vs. size and a smaller pond can more easily be kept cold in the summer with less flow. I.e. raceways have an incredible amount of trout due to the high flow rates. (About 50 lbs.per gpm and higher) In my flow through earthen pond I use a rule of thumb of about 12 lbs. of trout per gpm. E.g. my flow is 45 gpm so maximum pounds of trout is about 500.
Some things to keep in mind though:
1.) Have as steep as sides as possible to keep down warming of the water in the summer.
2.) Use bottom diffuser aeration but only run it at night if your temps are marginal in the summer.
3.) If you can keep temps in the 55 to 65 F. range in summer you are good to go.
Here's an example of how small a pond you can have to grow trout if you have enough ground water flow. It's one of the trout ponds at Crystal Springs Trout Farm in Michigan.
Another picture of 10 to 12 pound Golden trout:
Here's a couple of trout out of my 88 by 59' pond:
