Orthographic Size move between two values over time.

I'm farmiliar with moving an object over time but how do I go about changing a value over a set time.

For movement I've been using

float i = (Time.time - startTime) / duration; 
player.transform.position = Vector3.Lerp(startPos, endPos, i);

What I'm trying to do is get the Orthographic Size of my camera to move from 2 - 5 over 1 second.

Sounds like you're just looking for the "float" version of Lerp, as opposed to the Vector3 version. In this case, it's Mathf.Lerp, and you'd use it like this (with your start & end values set to 2 & 5).

float i = (Time.time - startTime) / duration; 
Camera.main.orthographicSize = Mathf.Lerp(startValue, endValue, i);