Particle size limited?

I'm trying to do a particle system that blows huge transparent fog particles over the screen but it seems the particle scale is automatically as big as it can get stopping me from making them any bigger. Even if I rescale the particle texture itself it will stay the same size in the engine. Is there any way to fix this?

By big particles I'm talking about 512x512 scale particles on a 1920x1080 game.

Bump!

First of all, this is very heavy on the performance if you have some 100s of these particles (more talking of mobile gpu heavy).
The particle system will soon get revised, when I've finished the gui.
In fact there are many things that aren't working at the time with particle systems.

Fair enough. I was actually planning to have very few particles and that's why they would be so big. I don't have any third party tools to make a good looking sandstorm effect for my scene so I pretty much have to use the particle system for it.