Gary. What country are you in? Here in NewZealand we use metric sizing. PFD & P&ID normally are drawn on a A1 paper. Often they are then only printed at A3 (so scaled 1:2) ISO text height apply. 1.8mm, 2.5mm, 3.5mm & 5mm. Most text in your drawing will be 2.5mm, titles/plant tag for say vessels 3.5mm. 5mm for Titleblock.
Are you using AutoCAD P&ID or AutoCAD Plant 3D to do your PIDS? If so you may be inserting symbols for use in imperial drawings (inchs).