Thank you for the clarification that it's per Kb. Unless I'm blind, I don't see that specifically made clear on the withdrawal page.
It's mentioned in the FAQ/on the blog.
Would be worth making it clearer
Probably left out there for the ease of it (not to overload the window), but could be a good addition.
and if at all possible, give an 'estimate' of that actual fee paid
As said, an estimate of transaction sizes is hard to make, due to the nature of the system.
The important variable for transaction speed is not the total fee paid, it's the fee per size ratio, so what you need to know for estimating your transaction speed is your per kB fee, not your total fee.
I don't think estimating the size and total fee would provide any additional value to the user (it doesn't change the amount you'll have to pay or the speed of your transaction).
It's pretty obvious to me that the fee displayed is per kb, but a small mention of it might be useful though.
Generally, it's a good idea to check how many unconfirmed transactions there are on the network, if there are, say, only 3000, you can get away by pointing that slider almost all the way to the left.
I once made a withdraw using only the bare minimum of fees (slider was all the way to the left) and it was confirmed the same day still.
Not a good idea in general, but usually it'll be fine.