Annie204 as others have mention not all speculums both metallic or plastic are finished to the same standards . As a rule the cheaper they are the more likely they are to have imperfections which can results in cuts , or scratches. Be sure you carefully examine your purchases to ensure a smooth finish free from rough imperfections .
As Amethyst suggest , ensure you use adequate lubrication and slowly open when the vagina. Don't try to stretch the vagina to fast , allow the muscle to relax and accommodate the opening speculum blades.
As you said
I'm thinking of getting a medium metal graves to push up my vagina. A small/virgin is used in my real exams as I have no sexual history only fingering.
I'm choosing a medium as I want it to hurt a bit and make me feel sore afterwards though I do get that a bit anyway from the small (i'll post that story elsewhere!). Will I be able to open a graves on my own?
You may not be able to fully open and lock a medium speculum , it does take time but you should be able to eventually do this over time . As exciting as this may seen , you don't want to cause actual injury to yourself and end up in hospital. An common mistake by novice users is to close the speculum fully before extraction , Never do this as it will result in pinching , which is very painful, always leave the blades open somewhat before removing the speculum .
You should be able to insert the speculum and open it by yourself , I would suggest a mirror to help you on the insertion , open and closing. Also have a towel handy or remove gloves if in use as it can be difficult opening or closing the blades with lube encrusted fingers or gloves .