Static Air Pressure is controlled by the Regulator. The air hose has no final effect on static air pressure.
Dynamic Air Pressure (pressure under flow), is affected by both pressure drop through the Regulator and pressure drop through the hose. Pressure drop through a hose is affected by Hose Inside Diameter (ID), Hose Length and Flow Rate (SCFM). The attached table provides a good estimate of that pressure drop at different flow rates through different size hoses.
1) A tool requiring 15 SCFM at 90 psi, is connected to a compressor with its regulator set to 90 psi using a 1/4" x 25' hose. Based on the table, pressure drop will be 29 psi, therefore only 61 psi will be delivered to the tool.
2) That SAME SETUP equipped with a 3/8" X 25' hose will only drop 3 psi, delivering 87 psi to the tool.
The Regulator pressure setting can be increased to compensate for the pressure drop as long as the Regulator is set to a value within the rated capacity of the tool.