Just a comment from an elderly DIY guy. Half a Bar is on the low side but is perfectly adequate for a height of 15 feet ( measured vertically from the level of the pressure gauge to the top of the highest point of the heating system). There must be a leak of some sort but it may not be visible water. I could be that the warmth of the system is evaporating the small leak before you can see it. It might even be an air leakage st the highest point of the system, but very unlikely.Top up the pressure to, say 2.5 bar (some systems have the pressure relief valve set at 3 bar) and then run your finger round all the radiator connections. If the pressure still drops, check your highest radiators to see if they need bleeding. I can't see that the size of the expansion vessel could be involved, as the sole object thereof is to prevent the systems's pressure relief valve blowing unnecessarily. If you have an old style isolating valve (wheel operated) the gland nut is often suspect and needs regular tightening. Best of luck.
One thought. When you had the new boiler installed, did you change the radiators? If you did, the fitting engineer is responsible for finding the leak. You could try turning off (at both entry and exit valves) ALL the radiators except one and seeing if the pressure drop continues. If not turn one radiator on at a time and check each time. Although the continuing nature of the problem suggests a pipe/radiator fault, try turning off the boiler, letting the water cool and then turn off the water feed and returns from the boiler to the system top up the pressure and see what happens.