I have a 100g tank that is 60", so 60" please.
More importantly, here is a feature that would set yourself apart from the pack and at some point someone is going to do it. Programmable Par Level. Allow the end user to enter the Par Level they want and the Substrate Depth and have the fixture automatically produce the given Par Level at the given Substrate Depth. It's just a user programmable dimmer. Of course each size fixture will have a maximum Par limit based on fixture LED design.
Total newbie talking here, but in concept makes sense to me.