reefs coud be as small as 1 gallon. but a basic rule is the smaller the tank, the harder that it is to care for. shoot for the biggest tank possible to start up.
75g is a real good place to start. You can have a nice reef in that sized tank as well as a decent variety of fish. A sump/fuge is also a real good idea. Big of one as you can fit under the tank. Hides equipments, adds water volume, makes water changes much easier, and if you put in a fuge with some macro its a great way to export nutrients and grow pods.
I agree with the rest and usually the numbers that I have always followed in being near perfect standard dimensions are: 10g, 20g, 40g (breeder), 75g, 125g, 180g, 300g, Olympic-sized swimming pool XD
I have a 46g reef the only disadvantage is I can't keep any large or even medium size fish just smaller damsels and such. The bigger the more options you have for fish.
My first reef was a 72G bowfront tank. (75g is the standard tank 72g is the bow). I had great success with the tank and only recently upgraded to a 120g tank.