i've heard people say litestep is about half the size of explorer.exe (can't remember tbh, it's been so long since i used it). Aston is dependent on the skin and normally is about the same.
Geoshell tends to be quite small too, but again depends how many bars you use.
If your a minimalist, geoshell is v cool.
Aston is great for eye candy.
Explorer is the best for maximum compatibility
My explorer.exe is about 15mb (VM size), and I don't think i've seen any shell much below 8mb in VM size (that was with my setup of geoshell) so really, how much difference does it make?