5 Gallon marine tank is challenging enough for people who know what they're doing but a first timer out is going to be an expensive lesson. Most Clownfish will stay under 4 inches (if they live that long) and it's true they don't wander too far from their Anemone.
If it were me I'd start off with a 20 gallons for a few reasons. If there is a small change happening in a 5 gallon tank it's a huge thing since it's so small of a volume. At least with a 20 it's a little more forgiving or gives you enough time to correct it without the tank crashing (killing everything). If you want keep inverts like Anemone the water has to be very good (low on nitrite and nitrate and no ammonia). It's going to be "work" keeping up with the tank. I feel it'll be cheaper just to watch Nemo on the telly.