Perhaps the easiest way to begin the process of predicting when a prospect will be drafted is to look back at the recent history of draft picks and see how the current year's players compare to those in the past from a physical standpoint. All the draft history data was found at Pro-Football-Reference and all the prospect measurements was found at Mock Draftable.
Mock Draftable has a very ingenious way of comparing prospects to others of the same physical attributes by creating a "star" graph with each category (height, weight, 40 time, etc.) on an axis by percentile within each position. I've had similar ideas to creating this type of star graph going back to 2009 so I'm glad to see that someone else thinks similarly and has actually created it. The general thesis is that this type of graph provides a general, well-balanced overview evaluation of many different numbers in a single graph. A perfect prospect measured on a star graph with 8 different attributes would look like an octagon, since hitting 100% of each axis would create the uniform shape.
So instead of starting by comparing prospect's physical measurements to those of recent past drafts, I'll let Mock Draftable do the hard work there and instead concentrate on something else in the beginning.
What I want to look at in the beginning is what specific measurements in each category (again height, weight, etc.) are better indicators of NFL performance, for instance do 6'3" tall wide receiver fare better typically than 6'2" tall receiver, etc. I saved myself from doing the hard work on grading how well players have done since entering the NFL by relying on the invaluable website Pro Football Focus (PFF) for that information. PFF has a team of game analysts that watch every players on every play for every team and who grade the player's performance as unbiased as possible. So to graph the players worth thus far, I collected each of their career values (how good they have been) and divided it by the number of years they have been in the NFL to get a "Career Value per Year" score.
Let's start with a simple graph, showing the prospect's age graphed against the Car Val per Year (all data analysis was done in SAS' JMP software):
As you can see from the smooth line fit, drafting younger players is typically better than drafting older players. There could be many theories as to why but I'll try to stay clear of causal theories in this statistical analysis.
Next I broke it down by positions. This allows you to truly compare apples to apples, since we're comparing one WR to another WR. This is the full breakdown for the WR position with some insights from the limited data set going back to the 2008 draft:
This shows the means (averages) of each year old when drafted, their height in inches and weight in pounds. There's a clear indicator that drafting younger WRs is better than drafting older ones but height and weight is less clear. There's no specific sweet spot for height and weight, but to answer my earlier question, yes typically 6'3" tall WR are better players than 6'2" players in the NFL. For a much, much deeper dive into WR measurables, the fantasy football site Rotoviz has done some great work thus far.