I don't remember where the rules for assigning size category to an object are stated explicitly, but it is generally the same as for creatures. You can use some guidelines from Animated Object monster entry (it has examples of objects of each size category) and from Equipment section of PHB.
Weapon Size: Every weapon has a size category. This designation indicates the size of the creature for which the weapon was designed.
A weapon's size category isn't the same as its size as an object. Instead, a weapon's size category is keyed to the size of the intended wielder. In general, a light weapon is an object two size categories smaller than the wielder, a one-handed weapon is an object one size category smaller than the wielder, and a two-handed weapon is an object of the same size category as the wielder.
Damaging part of an object
You can definitely damage a door since it is mentioned in Table: Object Hardness and Hit Points in Breaking and Entering If it is medium or smaller, you can use the Break spell, in any other case you can employ other magical or mundane means.
The same section has the following sentence:
Very large objects have separate hit point totals for different sections.
How to divide large objects into sections is largely up to DM, but I suggest treating any easily distinguishable object like a keystone as a separate object. Usually any 10 ft section of a wall is also treated separately.
Item's saving throws
I'll just cite the source:
Saving Throws: Nonmagical, unattended items never make saving throws. They are considered to have failed their saving throws, so they are always fully affected by spells and other attacks that allow saving throws to resist or negate. An item attended by a character (being grasped, touched, or worn) makes saving throws as the character (that is, using the character's saving throw bonus).
Magic items always get saving throws. A magic item's Fortitude, Reflex, and Will save bonuses are equal to 2 + half its caster level. An attended magic item either makes saving throws as its owner or uses its own saving throw bonus, whichever is better.
I've forgotten to answer about backpack. The description of the spell states that it can affect one object. Thus if you target a backpack, only the backpack is affected. Not its contents. Though the breaking of the backpack can affect its contents in turn (it can fall out and break if it's fragile, for instance).
And while it is common to treat a wall section as a separate object (have its own HP), it is not a separate object actually. So I'd say that you can’t target it with Break spell for two reasons: it is not a separate object and 10x10 ft object is larger than medium. Though DM is free to treat it differently.