Most divine foci are holy symbols...
Components on Divine Focus says
A divine focus component is an item of spiritual significance. The divine focus for a cleric or a paladin is a holy symbol appropriate to the character's faith. The divine focus for a druid or a ranger is a sprig of holly, or some other sacred plant.
A cleric or paladin or similar's holy symbol can be made from any one of variety of materials, while a druid or ranger or similar tends toward holly and mistletoe.
Holly and mistletoe cost and weigh a negligible amount. The game hasn't focused on those pretty much at all (except in very rare cases such as this magic ring) and, instead, provides all kinds of information about holy symbols. If you're really wondering about holly and mistletoe and not holy symbols, talk to the GM. Holy symbols get all the love.
...And a holy symbol is versatile...
All holy symbols weigh 1 lb. except those made of wood, which have negligible weight. However, I assume a cleric or paladin's holy symbol could be—I mean, if he really wanted it to be—as large as the character could drag: scaling many nonstandard things up or down will depend on the GM's house rules for doing so (that is, so far as I'm aware, no rules are provided for, for example, inappropriately sized holy symbols), but I tend to use the Armor for Unusual Creatures chart for resizing pretty much everything because it's simple and convenient.
It's probably safe to assume the typical holy symbol is, by default, equivalent to a light weapon for a creature of its size, so a one-size-category increase makes it one-handed, and a two-size-category increase makes it two-handed. For example, if he wants to channel energy with it or ward off a vampire with it or whatever, a Medium creature must use two hands to present a Huge holy symbol. However, I warn you: I must speculate here because I've never seen an inappropriately sized holy symbol be an issue despite there always being at least one divine spell caster in every party I've been in over the past several years, and sometimes I am the one playing that divine spell caster.
...But a holy symbol's not usually armor, a shield, or a weapon...
By default, armor, a shield, or a weapon isn't and can't be a divine focus. However, paying someone with the feat Craft Reliquary Arms and Shields 250 gp can turn any armor, shield, or weapon into a holy symbol divine focus (but, apparently, not a holly-and-mistletoe divine focus).
...And a divine caster must be able to manipulate the holy symbol and may want to be able to present the holy symbol, too...
Choosing a Spell says that
To cast a spell, you must be able to... and manipulate the material components or focus (if any).
This means a cleric can wear a holy symbol around the neck or as a ring or whatever as long as the cleric can manipulate the holy symbol while casting a spell (holy symbol manipulation, by the way, is done with the same one free hand that's also satisfying the spell's somatic components and preparing and manipulating material components—yes, that's a busy hand!).
However, actually presenting a holy symbol—for example, to channel energy or to ward off vampires—, I've always seen ruled as needing an empty hand (although not necessarily an empty hand capable of wielding a weapon—a holy symbol held in a hand that's also toting a light shield, for instance, is fine). (This makes the necklace—which the cleric can argue can be strongly presented using either hand—a more versatile choice than the ring which a cruel GM may rule is occasionally on the wrong hand.)
A holy symbol stored in a weapon's hollow pommel (a real thing with statistics in Pathfinder) can't be used until extracted, but if a foe thinks a cleric powerless after snatching the cleric's holy symbol, the foe could be in for a surprise next round. (It likely takes a move action that provokes attacks of opportunity to retrieve a stored item from a hollow pommel, but ask the GM.)
...and you probably can't just nail a cross to a shield (or whatever) then use the shield as a holy symbol
The existence of the feat Craft Reliquary Arms and Shields leads me to believe that trying to attach a holy symbol to armor, a shield, or a weapon probably just destroys the holy symbol, but you can run it by the GM and see if you can save 250 gp.
Options are identical for unholy symbols if you're evil.