Although the phrase a couple of has been well established in English since before the Renaissance, modern critics have sometimes maintained that a couple of is too inexact to be appropriate in formal writing. But the inexactitude of a couple of may serve a useful purpose, suggesting that the writer is indifferent to the precise number of items involved. Thus the sentence She lives only a couple of miles away implies not only that the distance is short but that its exact measure is unimportant. This usage should be considered unobjectionable on all levels of style.
few is imprecise, only defined as 'more than one':
1. Amounting to or consisting of a small number: one of my few bad habits.
2. Being more than one but indefinitely small in number: bowled a few strings.
n. (used with a pl. verb)
1. An indefinitely small number of persons or things: A few of the books have torn jackets.
2. An exclusive or limited number: the discerning few; the fortunate few.
1. Being of a number more than two or three but not many: several miles away.
2.-4. [deleted for brevity]
pron. (used with a pl. verb)
An indefinite but small number; some or a few: Several of the workers went home sick.
So, by these definitions, couple, few, and several could all mean two. Certainly not how I use it, or expect others to use it. (Also note that several is defined as "some or a few"!)
My rule of thumb has always been:
* a couple is exactly two
* a few is three, rarely four
* several is at least three but small, not exceeding five