I am using instant geyser with LPG. I was told the pressure will be low no matter what size I buy. I bought the 10 ltr one. It takes about 5-10 secs to get hot water and even at the lowest setting water gets very hot. Pressure is very good that we use it on two taps at the same time without any issue.
Only drawback is the high LPG cost. We went through 3 KGs in 5 days. We are 4 people getting showers every day.