Rollitup just posted an excellent write up on this subject,pot size does not have to be one gallon per foot or one gallon per month that the plant is going to live,that rule of thumb is correct but only to a point,it is what optimum conditions would be,these conditions can be manipulated by more frequent watering & a heavier regiment of fertilizers to make excellent use of smaller pots,i do not follow the one gallon per foot/month rule & get excellent results,infact i prefer smaller pots due to more frequent waterings,i use all one gallon pots & grow plants that reach 3tf high at harvest,with every watering comes fresh oxygen to the root mass,this is a very large step in the overall speed of the grow & will shave a week off of harvest time easily.
Something that alot of us experienced growers forget to tell noobs is this,pot size becomes less relevant to overall plant health & yeild depending on weather the plant is growing from seed or clone,clones are a more mature plant & need less growing medium over plants started from seed.
IMO its better for a begginer to start off with smaller pots,one of the biggest mistakes all new growers make is overwatering,this is much harder to do in smaller pots.