Only a bit of a shirt shows at your wrists and below your neck, which is why the collar is the most important part of a shirt. Consider poor Stanley Tucci here, unbecomingly costumed for his role in The Devil Wears Prada. The fit of his collar is classic, with what appears to be just enough room to insert a couple of fingers between shirt and neck and about a half inch of shirt above the back of his jacket. But the proportion is off - the points of his collar are too long for his head.
Proportion isn't difficult to achieve with a couple of simple rules of thumb. Men with round faces should wear longer collars and men with long faces, wider collars like the spread. A long neck requires a collar with a higher band and vice versa. The average man's straight point collar should be about 3" long, plus or minus a quarter. Larger men can add another quarter inch and smaller ones subtract it.
But for the collar, only a couple other elements are important for shirts that will live under jackets all the time. For one, the shirt tails should be long enough so that they stay tucked in. That requires about six inches of fabric below the trouser waist. And for another, the shirt cuffs should fall to the base of the thumb (if a man's jackets are cut to end at his wrist bone, this dimension will produce the requisite half inch of visible shirt cuff between jacket and hand).
The thumb actually gives the lazy shirt maker a way an easy way to achieve fit as he can (and usually does) simply make the sleeves a bit long knowing that the width of the hand will make the fit appear fine. This is so easy to achieve that when a man appears in a jacket that shows no shirt cuff, as did the Princes of England at the recent Combined Cavalry Old Comrades Parade, it merely points out the unforgiveable sloppiness of their tailor.