You don't need one for med school apps since the information is all recorded on AMCAS, if that's why you're interested, though your university's premedical committee may need one.
The general rule of thumb is that one page is usually most effective since a resume should be tailored to the most pertinent experiences/qualifications for the audience. However, this can be highly dependent on field and your age/experience. For medical students, you might expect more information on volunteering, leadership, and research that would make it completely reasonable to have a 2 page resume. After that point though, you may want to start differentiating between your CV and resumes for specific audiences.
You're welcome to message me a copy if you'd like me to take a look at yours.