Cervix positioning can definitely affect which cup you get. A person with a high cervix may prefer a long cup which could be easy to reach. A low cervix person could prefer a short cup which will not protrude outside the body. Typically, a high cervix person can wear whichever cup they want.
A person with a high cervix usually cannot reach the cervix with their fingers. This allows any cup of any length to be able to enter the vagina. However, a high cervix person getting a short cup can be bad for beginners who are not used to reaching for your cup. That can make removal difficult. A beginner with a high cervix should usually search for a long cup that fits their body build.
A person with a low cervix can easily reach the cervix with their fingers. These people can only use short cups. Also, people with short cervixes may prefer a bell bottomed cup because the cervix can steal capacity in their cup. If a person with a low cervix gets a long cup, then the cup may protrude outside the body and be uncomfortable. A person with a low cervix should search for a short cup with maybe a bell bottom.
Since your cervix position can change throughout your cycle, some people have a short cup for low cervix days and long cup for high cervix days. If this happens to you, it is recommended that you get a short cup so you can wear it whenever. When your cervix is high, you can bear down to bring the cup lower, then remove. The stem can also help in removal.
Height does not affect the location of your cervix!! I know many tall people who cannot use long cups because of their cervix. The best way to know where your cervix is is by trying to reach it with a finger.
Hope I helped!