According to present-day requirements, a full-sized, well balanced dog should not exceed 15 1/2 inches at the withers - the bitch being proportionately lower - nor should the length of back from withers to root of tail exceed 12 inches, while to maintain the relative proportions, the head should not exceed 7 1/4 inches or be less than 7 inches.
A dog with these measurements should scale 18 pounds in show condition - a bitch weighing some two pounds less - with a margin of one pound either way.
Balance - This may be defined as the correct proportions of a certain point, or points, when considered in relation to a certain other point, or points. It is the keystone of the Terrier's anatomy. The chief points for consideration are the relative proportions of skull and foreface; head and back; height at withers and length of body from shoulder point to buttock - the ideal proportion being reached when the last two measurements are the same. It should be added that, although the head measurements can be taken with absolute accuracy, the height at withers and length of back are approximate, and are inserted for the information of breeders and exhibitors rather than a hard-and-fast rule.
Amplification
It is as true today as it was in 1876, when this Standard was first adopted in England, that a dog of correct size will seldom exceed 7 1/4 inches in length of head. Any measurement over 7 1/4 inches normally indicates an over sized or long backed specimen. One should note the frequent use of the word "moderate" in the Standard. The key word is BALANCE. No part of the Smooth should ever call attention to itself when viewing the whole dog. The dog should be 15 1/2" SQUARE...when height at withers and length of body from shoulder point to buttock are the same, the ideal is reached.
Oversize Terriers are apt to lack type and character and extremes are to be avoided. Passing fancies such as extreme length of head or excessive rear angulation do little to promote the preservation of breed type. Judges, breeders and exhibitors must make every effort to prefer the Fox Terrier of correct size, spirited but tractable character, of substance combined with elegance and refinement.
They should be lively and active, but not hyperactive, and never timid, spooky or aggressive toward humans. All Smooths should have a gay, fearless temperament. The desire to "spar" or square off against and stare down another dog is very typical of males and shows them to their best advantage (Judges must exercise appropriate cautions). Some bitches do not display this behavior to the same extent.