Like many other areas of this hobby, there are no hard fast rules when it comes to tank size selection. You can make a case for almost any size tank as a first tank. Every size has good points and bad points.
The larger 4 foot long tanks from about 70 gal up are nice, but then you have the issues of the total investment, and they are a lot of work to initially set up. Anything larger would also have similar issues, but on a larger scale.
The mid size tanks from about 30 to 65 gal are a bit easier to set up. For example a 30 gal tank can be moved areound by one person. You'll need 2 people for anything larger. The cost to set tme up is also a lot less, since you can stick to hang on tank filters and skimmers. The downside is that you do have some limitations. Tangs are juat about out of the question, as are the large angels, and other large fish.
Small tanks from about 10 gal to about 25 gal can be real gems, but have even more limitations due to size. That 20 gal tank by the time you add live rock and live sand is only going to hold about 15 gal of water. You need to be very careful so that you don't overstock the tank.
Even smaller tanks can be done, but since they are small, a mistake that would be not too much a problem in a large tank becomes a disaster in a very small tank.
In all cases planning is key. Just about the worst thing you can do is go out and start buying equipment and livestock without having some idea where your going with the tank. If your new to the hobby, it never hurts to ask on RS about your plans. You may save yourself a lot of time effort and money.