Spaces etc do count towards the character count, returns count as 2 characters I believe. I used practically the same PS for MD + DO, MD one was ~300 characters longer, then I just trimmed it for DO.
if you've got your point across and answer the questin (AMCAS has a question while AACOMAS you can write about anything you want) then theres no need to add spaces just to fill them up. it will likely throw off the flow of your PS if you are trying to force things in.
spaces, tabs, skipped lines, etc. all count towards the character count. every time you push a key, it counts as a character.