First of all, size doesnt matter
Sorry. No it's just the size of the capsule and the amount you take won't help as much if the EPA levels aren't met. Because of the intention of omega 3 use not being for mood there is little chance that over the counter capsules will tell you the amount to take but its a certainty that 'one a day' products will not do the trick.
Rule of thumb is you will need well over a gram alone of EPA as that's the active ingredient. The more capsules you take to get that level the more geletin and other things you dont need are taken so you have to be careful.
Cuddlymoon, why don't you ask your GP to prescribe Omacor? That'll keep cost down and it's one omega 3 product that pdocs who are read up on such things tend to use. You could say it's to help with other things like aches and pains digestion etc but if you trust your GP you could say you're also interested to see if it supports quetiapine use. Many pdocs are now adding high dose omega 3 to anti-ps when treating schizophrenia to try and increase positive medication effect but not increase AP side effects. That way you'd stay on quetiapine as per pdoc treatment but you'd also notice if omega3 does in fact work then that would be a good time to see if you could start lowering seroquel dose once you have the build up of EPA in your system (takes many weeks for omega 3 to start showing signs of A/P or mood stabilising properties)
Just a thought.