One of the optional rules that I universalize across my GURPS adaptations is the logarithmic Basic Lift scale discussed in "Knowing Your Own Strength", from Pyramid #3/83.  In fact, I take it a step further and apply the revised scale to all four attributes (thematically doubling a given effect for each +3 to its score).  This condensed range (+6 for the human maximum) makes it far easier to choose appropriate attribute levels and not get lost in granularity.

When it comes to actually selecting an appropriate Strength score for a character, again I base it on what can be proven.  Sometimes there is anecdotal evidence consistent enough to judge, such as Conan twice being described as having the strength of three men.  Other instances, especially in cartoons, can vary wildly, so I opt to measure the musculature shown in concept drawings and whatnot.

At present, my best attempt at approximating the interplay between height, weight, and Strength is to start with a generic body scaled by the Hamwi formula, then adjust its muscle by the amount of cross-section area.  Because grip strength is a good estimation of overall lifting capability, I can subtract a character's wrist area from that of his forearm, scale by his height, and compare to ANSUR 1 data to learn how far above or below average the character's musculature ranks.