Determine the project for which the Pex tubing is being considered. Pex comes in standard sizes the same way copper or PVC pipe does. The most common sizes for Pex are 3/8 inch, 1/2 inch, 5/8 inch, 3/4 inch and 1 inch.
Size the Pex tubing needed by the amount of water flow needed. In a large floor where the tubing will be spaced a foot apart, the tube needs to be 1 inch. In a smaller floor where the tubing is closer together, smaller Pex will do.
Start with the larger size Pex coming from your main water source and narrow down the branches from there. 1-inch Pex at the main can be linked to 3/4 inch under the house and then 1/2 inch at the sink. Narrowing down the sizes uses less water.