3/4 pipe or 5/8 pipe for maximum pressure?

[mestalla]

I live in a mountain village. The water from a mountain deposit feeds 1.4 bar to my gas, bottle fed, Fagor water heater. The hot water feeds one kitchen tap and one shower. The pressure is not good enough to supply enough hot water (in Winter anyway). It is a 6 litre model so I intend to buy an Ariston 11 metre model. The distance down from the heater is about
twelve feet. I am going to renew the pipework. The taps are 5/8 and because they are buried, cannot be changed.When I change the down pipework however should I use 3/8 or 1/2 to get Max pressure.
I hope buying a new heater will improve things but I need all the advice I can get.....Thanks for any help

 

[doitall]

12ft head is around 0.35bar, the pipe size will not change that.

The larger the pipe the more volume you'll get

 

[mestalla]

OK thanks , then I`ll go for the 3/4 as I want more volume- But I am not relying on a 12' head because the deposit up top of the hill feeds the whole village at 1.5 bar. As a lot of pipework on the hot side normally is 5/8 I thought that would be more pressure like putting your finger on the end of a hose that is on, It is a very small supply out of the shower at the moment and all valves are ok and inside of heater clean.
Thanks for your interest.