I'm playing on a keyboard and mouse and whenever a mistress tells me to kneel and then stand back up i never really get the same height as i had prior do the different mistresses have canonical heights so i can get a rough idea of my height compared to their's?

Hello! The Mistresses are all generally between 5'9" to 6'5". They are mainly meant to be taller than most people. Yuki-Onna and Nega-Yuki are about 5'5".

Are you using the spacebar to raise yourself back up? 

Yes i use the spacebar.