ligation - stick 3 pieces together - (Sep/15/2004 )
Linear, unligated plasmid will run at the molecular weight, will correspond to marker. Ligated, closed plasmid will run "anywhere", normally somewhere higher than the linear form. So if you mean compare size by base pairs, no it's not possible. If you mean compare size to determine whether the plasmid closed- then yes, it's possible.
The gel percentage- depends on the size of your plasmid, the bigger the plasmid the less concentrated the gel- to have good resolution about the right area. Start with 1%.