I just want to go through this and make sure I have everthing straight.  
From Tome and Blood, page 56 under dragon disciple.  
"Ability Boost:  As the dragon disciple gains levels in this prestige class, his ability scores increase as noted in Table 3-8.  These increases stack and are gained as if through level advancement."
under Dragoin Apotheosis:
"...and he gainst an additional 4 points of Strenght and 2 points of Charisma."
So over all ten levels of the prestige class Ability Boost and Dragon Apotheosis grant a total of +8 str, +2 con, +2 int, +2 Chr.  
"Enlargement:  At 5th level, the dragon disciple's size increase one step, from Small to Medium-size or from Medium-size to Large."
This very specifically says there is a size increase, not that the dragon disciple gains some of the benefits of a size increase, no?
From the MM, page 12 under Size Increases:
"A size increase affects a creature's ability scores, AC, attack bonuses, and damage ratings as indicated on the following tables."
taken from the following table. 
Old Size..........New Size.....Str.....Dex.......Con.....Natural Armor.....Ac/Attack
Medium-size....Large..........+8......-2..........+4.........+2....................-1
From the MM, page 5 under Size and Type:
"A creature's size also determines how far it can reach to make a melee attack and how much space it occupies in a fight (see Face/Reach, and also Big and Little Creatures in Combat, page 131 in the Player's Handbook)."
excert from PHB page 131 table 8-7: Creature Size and Scale:
Creature Size.....Example Creature.....Natural Reach......Face
Large (tall)**......Hill giant....................10 ft....................5 ft.x 5ft. 
**Tall creatures are upright. 
From what I gather this is what comes from the size increase.  +8 str, -2 dex, +4 con, +2 natural armor, 10ft reach, -1AC/attack.  
So total over the course of all 10 levels of the prestige class a dragon disciple would gain the following.  +16 str, +6 con, -2 dex, +2 int, +2 chr, +6 natural armour (+4 from the PrC, +2 from size increase as this should stack as this bonus would go to creatures that already had natural armour if their size increased), 10ft reach, -1ac, -1 attack.   
Now did I miss anything, did I leave anything out, or did I get anything wrong?
Can anyone show me OFFICIAL eratta that contradicts this?
[edit:  tried to make the tables a little easier to read]