Just replaced last 120 feet of home water supply line. Old line was 1" galvanized, new line is 3/4" PEX. Lines are six feet underground here in our mountain location and must feed upstairs bathroom along with the usual downstairs fixtures. Most lines under house are 1/2" copper. Have experienced unacceptable pressure loss with new 3/4" line. Now is time to fix as haven't covered trench yet. Do I need to go back to 1" line and if so will 1" PEX give us our previous pressure? Prior to new line, pressure was always very good. Water supply is from spring 1/2 mile above house, gravity supplies pressure with 2" poly pipe main line from spring.
As you now realize, going from 1" steel pipe to 3/4" PEX is a big step down. 1" steel pipe has an actual ID of 1.05" and 3/4" PEX has an actual ID of 0.68". At a flow of 8 gallons per minute, the pressure loss for 1" steel is 1.9 PSI/100' and the pressure loss for 3/4" PEX at that flow is 12.3 PSI/100'. What you did was the equivalent of stepping down approximately two sizes.
1" PEX will still give less than your previous pipe but it might be acceptable. The pressure loss of 1" PEX at 8 GPM is 4.2 PSI/100' and the pressure loss of 1-1/4" PEX at 8 GPM is 1.6 PSI/100'. You will have to decide what you want as far as price vs. performance.
EDIT: You might consider if another material would be suitable, such as 1" SCH 40 PVC. 1" SCH 40 PVC has losses similar to 1" galvanized.