How safe would it be to start a 20 gallon salt water tank?

I know the bigger the better. But, would i really be in for a tough ride?

START OUT SMALL. Learn the chemistry and then move up.
Get a cube or a nano. They actually take care of them self.
Plus they come with everything you need.
built-in lights, skimmer, and refugium/sump on the back

I agree with both nitty and tango. they are both correct in their own aspects. not to down nitty in any way, but the learning curve is there regardless of tank size, i prefer larger because i can mess up and not kill everything in my tank as easily but, for convience, they come equip with everything you could possible need for a SW tank built into them

if you are wanting reef-corals or corals and fish etc- if you go small get a nano, they are designed for reef setups or something like a mantis shrimp. more water changes and testing involved though even thoughthey are much more stable that turning your old 20gal into a SW tank, still require more hands on work. if you get a standard size tank i say no smaller that 50-55 and stay away from tall tanks like hex tanks they look good but do not provide good surface area or the real estate for SW fish to swim around. 55 long you can house between 4-12 fish depending on their max size, a 55 hex or tall 2-4 fish of small size and they have to occupy different zones of the tank. you cant have 4 open water free swimmers, like some chromies and clows and firefish, not enough room for them all to swim. If you want fish only then i would say go big, fish are messy and the tank will stay much cleaner if its larger. I know nano's are supposed to be beginner tanks but I disagree(IMO) you have to know what you are doing and you have to keep a fairly tight maintenance schedule which is not the habit of the average beginner sorry to say lol I know I was 100 % int omy tnak when I started and i made alot of mistakes that probably would have wiped out a nano 3-4 times..I had an accident with bleach that killed all of my fish most of my corals made it but I caught it early thank God for my girl waking me up..if that was in a nano im sure it would have been a total loss but to each is own, I LOVE nano set up's i just know im not the one to take care of it properly so I went big and I love that one too lol. Id say this, if you know you want salt...and you know that you would like to have one of those nice sized display tanks like LFS's have...then go big it is what you want so you might as well learn what to do on a size that you are wanting to have in the long run because whether you go big or small....your going to learn the same things just in different amounts...

NANO-
pro's-ease of use, everything comes built into them, small size- less LR and LS needed
cons- more maintenance over all due to small size, price, temperature tends to run high after a while more so depending on the manufacturer of the nano, more likely to loose everything invested if tank crashes or power outages or something like that, the size just doesnt hold oxygen long like tango said, nitrates spikes and things like that will really affect the nano

STANDARD TANK-
pros- more stability in the system which give you more room for error and less crashes. once you get started with SW fish and coral you WILL want more so you have a fair amount of room to fill up. easier to maintain and more forgiving on personal slips(forget to do water changes or to change out filter media dropped too much food in) price(you can usually get a 55 combo for a lot cheaper than a nano) filtration options are greatly increased, sumps, HOB filters, canister/drilled systems etc. real estate(some items need room and depending on what you get, you can have more diversity in a larger tank my buddy has a nano but it houses a few coral frags that he can not let get big and 2 clowns..thats all that will fit in it comfortably
cons- more of an investment needed as far as LR and LS goes, size, if you are cramped for space, cost of running(bigger stronger lights and pumps than on a nano) amount of water to change in water changes(55 gal your gonna end changing roughly 15 gal give or take, nano 3-5 depending on how you are stocked.