To check your inside leg length please lie jeans flat & measure from the crotch to the hem
To check the waist size on jeans you already have please lie them flat, don't bring the front of the waist up to match the back of the waist
Lie them flat & measure across the very top inside the waistband that is & double the measurement to get your waist measurement
Different styles of jeans sit differently & the rise can make a difference too