You do not gain a size templet You gain just what the spells say you do size mod is written into the spell and that is all you gain
From the beast shape spells
Small animal: If the form you take is that of a Small animal, you gain a +2 size bonus to your Dexterity and a +1 natural armor bonus.
Medium animal: If the form you take is that of a Medium animal, you gain a +2 size bonus to your Strength and a +2 natural armor bonus.
Tiny animal: If the form you take is that of a Tiny animal, you gain a +4 size bonus to your Dexterity, a –2 penalty to your Strength, and a +1 natural armor bonus.
Large animal: If the form you take is that of a Large animal, you gain a +4 size bonus to your Strength, a –2 penalty to your Dexterity, and a +4 natural armor bonus.
Diminutive animal: If the form you take is that of a Diminutive animal, you gain a +6 size bonus to your Dexterity, a –4 penalty to your Strength, and a +1 natural armor bonus.
Huge animal: If the form you take is that of a Huge animal, you gain a +6 size bonus to your Strength, a –4 penalty to your Dexterity, and a +6 natural armor bonus.
Small magical beast: If the form you take is that of a Small magical beast, you gain a +4 size bonus to your Dexterity, and a +2 natural armor bonus.
Medium magical beast: If the form you take is that of a Medium magical beast, you gain a +4 size bonus to your Strength, and a +4 natural armor bonus.
Tiny magical beast: If the form you take is that of a Tiny magical beast, you gain a –2 penalty to your Strength, a +8 size bonus to your Dexterity, and a +3 natural armor bonus.
Large magical beast: If the form you take is that of a Large magical beast, you gain a +6 size bonus to your Strength, a –2 penalty on your Dexterity, a +2 size bonus to your Constitution, and a +6 natural armor bonus.
And that is the whole list of size mods you gain from beast shape. Myself I loved shapeshift from PHB2 and so love the changes to wildshape myself