It can be said with all honesty that the better you know the rules, the more you can bend them before they break. Once you've run a large, successful marine tank, you'll find the greater challenge of a small tank much easier to manage. As a beginner, you'll find that the bigger your marine tank, the better. The temptation to cram lots of colourful fish into a tank is generally too much for novice marine keepers and is the reason for most failures as many people just can't resist adding that 'one last fish.'
The heart of any successful marine aquarium is liverock and water movement. With this simple maxim in mind, a tank large enough to hold plenty of liverock and still have room for water is normally going to be of around 80-100 litres in capacity. Tiny tanks can be run with care and there are plenty of miniature fish and shrimps that can make a 'nano reef' a wonderful slice of nature in your living room. As small tanks are liable to rapid swings in temperature, salinity, etc., they need much closer monitoring. After all, the phrase 'a drop in the ocean' sums up the stability of the environment that these creatures come from.
If you fall under the spell of Nemo, a good guideline is a tank of at least 80 to 100 litres, as much liverock as you can afford, a protein skimmer and much of the equipment needed for tropicals. The systemised packages available are probably your best option and are designed to be user-friendly. Bear in mind that although messier than corals and other invertebrates, the fish themselves are often easier to keep. For example, virtually all the common clownfish you will see are captive bred in hatcheries, love people and fish food and have no idea that anemones even exist. By contrast, anemones need perfect water quality, high-intensity lighting and expert care in order to thrive.