I like the idea of a full suspension build on a custom frame using 20x2.5 inch bmx tires. Reasons why: the larger the wheel, the less rpm is needed to move it at the same speed, which is useful if you have a gear increase but unless you're running a hit and miss engine you need a gear reduction.
Lets say the actual diameter of a 20x2.5" tire is 22", on a two stroke kit a 24 tooth sprocket will hypothetically get you to 36 mph at 6k rpm, where as a 26x2.0" tire would need a 32 tooth sprocket to reach a similar speed at the same rpm. Why is this important? Lets say this build is using an internally geared hub in the rear wheel, these can only take so much torque without risking damage, thus a smaller reduction then becomes advantageous.
Even a more traditional build can find a smaller wheel advantageous, maybe a disc brake adapter is used, supposedly a 36 is the biggest sprocket that can be comfortably fit behind a 203mm brake rotor. A 36 tooth sprocket on our fat bmx wheel will give us a higher reduction, thus more torque at the same power, thus steeper hills and heavier riders can be accommodated.