Can one set how many confirmations MultiBit requires for a transaction to be accepted? Is there any way for users to specify the number of confirmations for a transaction?
If not, how many confirmations does Multibit require to complete a transaction?
And does the required default number of confirmations vary by size of the transaction?
From your side, an outgoing transaction are completed the moment you send it. However, it's up the receiving party to decide how many confirmations they require before they consider it received. The usual rule is 3 confirmations. 

I have never used Multibit, so in regards to incoming transactions, I have no idea.