Generally agree with Jules, but be guided to a certain extent by your pump connection sizes. Many people use TAMs etc with 1" connections, so 22 mm will be OK, but with bigger pumps consider using 28mm at least for the suction.
The other reason for 22mm being a common choice is that benders for 22mm are readily available and easy to use. Start trying to bend 28mm and you're looking at a serious bit of kit and muscle to bend it ... bent and swept bends being preferable to solder-on elbows.
If looking at steel pipe, have a think around car exhaust pipe. I'm considering this for my long awaited processor rebuild. For the pressures we use, you don't need a heavy schedule pipe. Nice smooth bends can be bought at reasonable prices, many different diameters are available and you could even use stainless if you wish. Down side is that all joints will need welding and you'll have to acquire flanges for points where the pipework will need dismantling.
