Can I use an instant bridge as an improvised weapon?

Assuming 40+ strength with shikigami manipulation, could one use an instant bridge as an improvised weapon? Penalties are fine.

I've built a goliath druid / hinyasi brawler, so I do have a secondary question: Would the bridge grow with you if you were to wild shape into a huge-sized humanoid giant?

I'm a relative novice at Pathfinder, so any help or advice is much appreciated.

Yes, if you're large enough

The rules say:

To determine the size category and appropriate damage for an improvised weapon, compare its relative size and damage potential to the weapon list to find a reasonable match.

Your instant bridge has a length of 30ft and a weight of 300 pounds.

Checking the size table, I see that a Huge (tall) creature has a natural reach of 15ft, which means that a Huge reach weapon would have a reach of 30ft. So we might compare your bridge to a Huge polearm, such as a Huge bec-de-corbin, which would have a reach of 30ft and a weight of 48 pounds.

(Weapon weight doubles when you go up a size category, which seems wrong -- shouldn't the increase be a factor of eight? Rules As Written says it doubles, though.)

If your DM cares more about your weapon size than its weight, your bridge can be modeled as a Huge polearm. (If your DM tries to find an appropriate weapon to model a 300-pound weight, they're going to wind up larger than Colossal.)

But not in the way you want

But it sounds like your real question here is: "hey, can I use the improvised-weapon rules to get around the normal restrictions on weapon reach?" The answer is: probably no, because your DM should model your improvised weapon as a weapon with the same reach.

Weapons do resize

You've also asked about weapon resizing. A goliath druid shifting into a Huge giant uses the giantform II spell, which is a transmutation(polymorph) effect. The transmutation rules say:

When you cast a polymorph spell that changes you into a creature of the animal, dragon, elemental, magical beast, plant, or vermin type, all of your gear melds into your body...