In expansion the space in between the particles increase, not the particles, however does that still apply to plasma? and when a gas turns into plasma do the particles get smaller?
When a gas turns into a plasma, each atom or molecule loses one or more electrons. These electrons are a lot smaller than atoms or molecules, so in this sense many (about half) the particles in the plasma are a lot smaller than the atoms or molecules of the original gas.
An atom which has lost an electron is also somewhat smaller on average than the neutral atom, but this is not such a big effect. For atoms in group 1 of the periodic table is a moderately substantial effect.
Although the particles are thus smaller, as measured by their distribution of mass and charge, nevertheless their effects reach out to larger distances because now they are charged, so they each have a substantial electric field around them, and a magnetic field too if they are moving. When considering things like collisions and scattering of light, this is usually more important than the size of the particle itself.