I own and use both sizes you referenced as choices.
Since you in a teaching situation, I would stay with the 4.5".
I consider the 7" as more of a production size, compared to a 4.5, where production speed is required.
If you are teaching inexperienced students how to properly and safely operate a grinder, in my opinion, the 4.5 would be easier to learn on.
Any grinder can cause serious injuries in inexperienced hands. And in experienced hands as well sometimes. Their reputation for potentially causing injury is well earned and real.
I see no need to teach technique, with a larger, heavier, and more powerful 7" unit, when the 4.5 will be far easier to gain both experience and confidence.
I am assuming this choice is based on students with very limited grinder experience.
Learn on a starter size, then after gaining experience, perhaps graduate to a larger size grinder as production needs mandate speed.
Bill