You dont need Physical measurement certificate at initial stage,However its required when you qualify written test.If you are doubtful about your physical measurements then you may Try your local DHQ hospital.Normally they don't do it unless they are asked by specific department,2nd option is visit your distt police office's OASI branch and tell them you want to get your self measured for your own sake.These people have vast experience of measurement as they perform it all the time during recruitment process in distt police.