Can you increase a shield size category?

[Vaeshen]

Reading over the boards the shield options seem very limited currently to only Sturdy shield if you want to use them for shield block.

One of my characters is very defense focused and does not mind the drawbacks of a tower or fortress shield, however it feels like I have to choose AC or the ability to shield block.

Can you increase the size of a shield like you do weapons. From medium to large. Like a large dagger is a medium short sword? In doing so make a large Sturdy shield the size of a tower shield.

Even if they double the bulk a large sturdy shield would be 2, compaired to a mediule tower shield with bulk of 4.

Huge sturdy shield bulk 4
Fortress shield bulk 5

Then add the Str and movement restrictions instead of being clumsy.

Does this sound like a viable work around?

Or preferably allow shifting runes on shields.

[Squark]

The problem you're dealing with was addressed in the remaster with Reinforcing Runes

[Perpdepog]

I'd also point out that nowhere in the rules does it talk about large daggers being treated like longswords. The Items and Sizes rules explicitly state you gain no real benefit from using a larger weapon, in fact.
You might be thinking of how different sizes of weapon worked in 1E, where that sort of thing did function that way, and larger weapons had larger damage dice.

[Ruzza]

You can, of course, purchase and carry a larger shield. It's just that, outside of houserules and homebrew, it doesn't do anything.

[Gisher]

As others have said, larger size weapons and shields don't grant any benefits in PF2 (with the singular exception of a giant instinct barbarian wielding a larger size weapon).

While you can't put a shifting rune on a shield, there are the Sapling Shields that transform between a buckler and a tower shield. Adding a Reinforcing Rune to one of those makes it even tougher.