Thinking 4.5 inch. That is an interesting idea about purchasing online. Do you know of the process to then register it in state?
The firearm can't legally be sent to you and they will not do this unless you are an FFL. Most sites have the ability to put in your Zip code and then you pick the local FFL. They will get the paperwork necessary from the FFL of your choice (I only encountered 1 site that wanted you to do the leg work. I said F that and paid $5 more on another site). When the firearm arrives you walk in fill out all the legal paperwork, pay the transfer fee and pick it up.