physics_particle_get_radius()
返回： Real（实数）
With this function you can find out what the current radius (in
pixels) is for particles in the physics simulation (you can set
this value using physics_particle_set_radius()).
if physics_particle_get_radius() < 32
{
physics_particle_set_radius(physics_particle_get_radius() + 1);
}
The above code will check the current radius of the particles in the simulation and if it is less than 32, then it will increase their size by 1.