Ok: What does all this mean?
The top table shows a segment of heights and weights for characters and the bonuses to hit them (HTH/RNG), their Reach Bonus, and their CON Bonus (if any). The %'s next to them show how this combination of modifiers fared in our tests. I tested basic 50/50 character splits with these and did not specifically test the Size characters (who were also 50/50 but were not 'normal' test characters).
The next table shows the actual Size Tests. The "red" numbers are the bonuses to STR and BLD for the listed number of AP spent on Size (so for the first one I spent 8 AP on Size and ran that character against the 16pt Herd with a sword and won 51.92% of the time).
The Blue numbers are what I estimate the actual costs would be in JAGS (it's sort of an approximation of the above numbers).
What You Do
What you do is buy Size at the "blue costs" and then, based on your character's Total Build you apply the above table's modifiers to the character. So let's say I want to spend 36 AP on Size for a character. I decide the character will be Unarmed.
On the Unarmed (blue) Table I spend 8 AP on L1 (+4 STR, +15 DP, +15 BLD) and then 3 levels of +1L for 24 AP (+12 STR, +30 BLD, +30 DP) ... then I spend 4 AP to round it out (+2 STR, +8 BLD, +8 DP). The character gets: +16 STR, +53 BLD, +53 DP.
If my character was 12 BLD to start with they are now 65 BLD. That's 975 lbs. On the above table that is above 800 lbs and below 1000 lbs. The character gets +2/+3 To Be Hit,+1 Reach to all HTH attacks, and +1 CON.