The size of tank you should start with is greatly influenced by your budget. Once we know a budget, and what kind of tank you want to keep then we can suggest a size that will suit that.
So here's a few questions to help:
What do you want to spend for initial startup?
How big is the space that you want to put the tank?
Do you want to keep corals?
If so, what kind? Softies? LPS? SPS? (do a Google search if you need to)
What kind of fish do you want to keep?