I'm 5'8" and 143 lbs (if I'm holding a 3 lbs weight)...I have been told I have the physique of a middle schooler.
A ton of your capability for lifting is predicated on your technique, followed by strength, followed by how many firemen show up for lifting assistance.
If you have bad technique for lifting/moving, you will hurt yourself and/or your patient, and will have a hard time moving people. Form, power, then speed.
That being said...it is only rarely that you have to actually lift somebody to any significant height. More often you're lifting the stretcher to some height. I suggest you review videos of proper lifting technique for both the stretcher and people/objects prior to making any other attempts.
I would also follow the sage advice of DEMedic and 46Young w.r.t. appropriate weight training. This will help too.