How can I specify transaction fee size when creating a raw transaction?
The fee is the difference between the inputs and outputs, so you can specify the fee by leaving that amount unaccounted for in your transaction. 

Be careful though, this has lead to 200BTC transaction fees in the past when someone forgot to include a change address: http://www.reddit.com/r/Bitcoin/comments/1lb5my/asicminer_refunds_the_accidental_200_btc/