Think about whether you really need a big bore pipe.
Using pipework larger than necessary will lose more heat overnight. It's easy to envisage the overnight cooling - The water in the pipework has been heated up during the day by some of the energy collected by your panels. No matter how good your pipe insulation is, the water in the pipework will have fallen to ambient temperatures overnight.
A similar scenario happens during the day, and a large bore pipe will typically lose more heat during daytime compared with a smaller bore pipe. E.g. Say during a brightish spell between periods of heavy cloud cover, your panels manage to heat 3L in the panels to 4 or 5 degrees warmer than the water in your tank. The controller will switch the pump on to bring the heat into the tank. And the more volume in those pipes in the first place, the less of the heated water makes it into your tank. Some of that heated water will stay in the pipework and not make it into the tank, and when the next batch of heavy cloud comes over the heat in the pipework will be lost. So, narrow bore pipework means you can make the best of marginal weather conditions.
I did a few example sums a while back and concluded that oversized pipes could easily lose 5% of the annual energy capture of your panels.
Now there are other considerations re pipe sizing - It isn't just a simple of case of 'the narrower the better'. E.g., very narrow bore pipe will give your pump a hard time, and, if you are relying on thermosyphon (gravity feed) and not using a pump/controller, then you have no option but to use large bore pipe (perhaps 22mm) to allow the thermosyphon to work.
Most DIYers use 10mm pipe on a coil. Comes in 10m,20m, 25m lengths and not too expensive. Bends are easily done by hand. Most commercial installers use expensive stainless corrugated DN16 flexi-pipe, primarily because it's handy, and they won't be thinking too much about energy efficiency.