The only thing stopping you from making it as big as you want is your budget, and how many fish you want.
You need to set out on the lawn what shape and size you want, use a hose to get the shape if its not square for a couple of days and it will leave the grass marked for you. You can make it as deep as you want but usually 3 foot is enough. Step the sides so you can put ledges in and you will need to work out how much liner you need to overlap by a fair amount so you can get stones to hold it down. A preformed pond means you dont need a liner, just dig out the shape of the pond, but i think you still need to put sand underneath it.
Next work out how many gallons it will roughly hold. Goldfish need around 50 gallons each but in a big space it can be flexible.
Next, get your pump and filter and get those in to position. You need the pump to turn over the volume of the pond once every hour. You might want to consider a waterfall of some such now as well.
Get a shovel and start digging it out and then line it with either carpet or sand and underlay. Carefully place the liner in to the centre and then use bricks to hold the edges up to stop the liner from falling in.
A hose is needed to start filling and stay in the bottom in bare feet and use your toes to press the liner into the corners. As it fills, gently let the edges go and the water will push the liner in to place. Corners are the tricky bit, you need to fold those over so you dont get big gaps. A lot of little folds are better than large folds.
Place the pump on the liner at the deepest point and get it all rigged up to the filter. If you dont want to bother with a waterfall then you can semi bury it in the soil in amongst plants and have it directing the outlet straight in to the pond.
Let it run for a couple of days then add bacteria to the filter and slowly add fish over a few weeks. Test your water after day 5 to check for ammonia and hopefully thats it. Ours took weeks to plan but only two days to dig and fill. Its around 600-800 gallons and has 6 goldies in it. It is hard work but worth it
Thats how we did our pond anyway, but im sure there are a lot of alternatives. My dad for example dug a hole, lined it and threw a couple of goldfish in.