Schedule 40 PVC pipe is code approved for water supply lines and essentially there is no difference between gluing 1" and 2" pipe, however PVC pipe is far from being the best choice of material.

PVC is made in rigid 20' lengths which means you will have a joint every 20' and at any change in direction. When you consider that the leading cause of leaks results from joint failures it is then obvious why we prefer to use PE or PEX continuous roll pipe, with PE being the material that is most commonly used. It should also be mentioned that while all pipe used in direct burial applications should be bedded with loose sand, none the less, the continuous roll pipe is much easier to install because being flexible, it will tolerate minor changes variations if the depth of the trench. Also, PVC tends to dry out and get brittle with age, therefore it does not tolerate rocky soils or roots as well as the roll materials do.

Now, contrary to public opinion, there is no guess work in determining the correct size of the line. Before we could determine the size of line for your application we would need a lot more information.

When determining the correct size of the water supply line we first need to have a list of all the fixtures in the structure to determine the total demand. By example, I have used the code tables to determine the total load for a typical house with two bathrooms and code minimum fixtures in each room. (from IRC Table IRC-T 2903.1)

KITCHEN:
sink..........................2.5gpm
Dishwasher....................2.75gpm

Bathroom #1
Lavatory.....................2.00gpm
Tub/shower...................4.00gpm
W.C. (tank type).............3.00gpm

Bathroom #2
Lavatory.....................2.00gpm
Tub/shower...................4.00gpm
W.C> (tank type).............3.00gpm

Laundry tub..................4.00gpm

Hose Bibb....................5.00gpm

Total load..................32.25gpm

In order to minimize the effects of pipe wall erosion code limits the "Velocity of flow" in copper pipe to 8ft/sec and in plastic pipes to 12ft/sec. For that reason the pipe friction head loss tables generally list both the velocity of flow and the pressure loss per 100 ft.

Code requires that we must provide a minimum of 45psi at the main water shutoff valve where the water enters the structure however we also want to minimize the material cost of the pipe so we now need two more factors before we can determine what size the line must be.

We need to know the average working pressure at the prime mover, whether that is at a well head or from the municipal water main and we need to know the difference in elevation between the location of the prime mover and the main water shutoff valve to determine the amount of vertical static head gain or loss.

By example, if your house is built on a hill and the location of the main water shutoff valve in your house is 50ft higher than the location of the municipal water main you would have a vertical static head loss of 50' x 0.434psi/ft vertical or 21.7psi vertical static head loss. (if your house was 50ft lower than the municipal main it would gain the 21.7psi, while if you have a flat run VSH = ).)

The friction head table jumps from 30 to 35gpm so we can round off to the nearest number, which in this example would be 30gpm.

At 30gpm you 1" line just barely meets code standard with a velocity of flow of 11.12ft/sec and a pressure loss of 18.35psi/100ft or a total pressure loss of 36.70psi.

An 1-1/4" line would yield a velocity of 6.42ft/sec and a pressure loss of 4.83psi/100 for a total loss of 9.66psi.

an 1-1/2" line yields a velocity of 4.72ft/sec and a loss of 2.28psi/100 or a total of 4.56psi.

A 2" line yields a velocity of 2.86ft/sec and a loss of 0.66psi/100ft for a total pressure loss of 1.32psi

In this example, if the municipal main pressure is greater than 54.66psi you could meet code standard with an 1-1/4" line and if the municipal main pressure is greater than 49.56psi you could meet code standard with an 1-1/2" line.

A word of caution here. When calculating the fixture demand the code requirement for a shower is 3gpm, however for the purposes of estimating both supply and DWV lines a shower is not classified by the number of shower enclosures, but rather it is determined by the number of shower heads that can be operated simultaneously. If you have a super shower with a regular shower head, a hand shower, a rain head and body sprays you must calculate at the rate of 3gpm for every shower nozzle that can be operated simultaneously.

I have seen many a custom shower failed on code inspection because they installed the standard 2" drain line, but when the inspector computed the DFU load for all the showers the drain line was not big enough.

Here is a link for plastic pipe friction head loss tables that you can download in PDF format.

http://www.eljay.com/pdf/friction%20loss.pdf

Is there a reason you are replacing the pipe? You mention a problem don't say what it is.