Hi, Now is a GREAT time to figure the entire system. How much water? How much filtration?
Here's a general rule for filter pit size; About 1/3 the size of the pond! Yikes! Yep. I had to scale the pond and pit to fit my yard. Also, I had to factor in the amount of filters/gallons the pit could hold and had to scale to pond accordingly. My pond is 3,400g at 12X8X5, so my pit is 1,077g and 6X6X4.
So to have a filter pit 1/3 the size of your pond, a nice bunker about 14'L x 8'W x 5'D would be perfect. Sound crazy? Start drawing a filtration system for 15,000 gallons of water and you'll see how much room it's going to take. With sieves instead of the settling chambers you can decrease the footprint alot. Settling chambers for 12,000gph would be nearly 1,200 gallons!
What I'm saying is, you're on the right track. Don't dig yet!
What's your budget? Do-it-yourself? Going to dig with a shovel? Liner? Gunnite? Poly? 55 gallon barrels are free, sieves are $700-$1,000.
looking forward to seeing this project progress,
good luck,
steve