Noob looking for a new double din head unit !!!! help me

[obito]

Now, I'm looking to replace the OEM head unit with a decent double din head unit.

What is the different between 6.1 inches and 7 inches? Which one perfectly fit with the OEM slot? Do Appradio require a cable to connect to the phone or today we could use it without the cable? I prefer wirelessly connection.

My needs are be able to plug in and play Apps from the phone ( I think it's called Appradio), mirror link would be perfect, be able to control things over the display for example skip, play, and pause Spotify, and be able to support new Iphone6 or should I wait for the iphone 6 to come out and find the one that compatible (for this one I do not sure that can we just change the adapter for the wire and we are good to go or we need to look for official compatible model), dont want the GPS function because I wanna use the GPS thru the Appradio ( the one with GPS navi tends to be more pricey than the other),and HD radio is not necessary lol.

Ps. I dont need DVD player since Im not gonna watch anything ><

and moreover What happen to the outdated Headunit? For example, if it said compatible for 4 and 4s not 5, Will iPhone 5 still able to use functionally or people just upgrade their Headunit?
Thank for helping