information about the exact fee before transaction
Keep in mind that the ultimate effect of the fee amount, the speed of confirmation, is fundamentally very inexact. It may not be worth the trouble to aim for exactness here.
I think though that Mycelium displays the exact fee and changes it when you change the priority level.
This is correct... after you put in the amount you wish to send, on the "confirmation" screen... you get to see what fee the system has calculated for your transaction before you send it... you can also cycle through the fee levels (Low-prio, economic, normal, priority) by tapping the box...
see here:
http://imgur.com/a/35oLUIt doesn't give you the exact transaction size... and it doesn't give fine grained control... having said that, I've used "low prio" for all my sends so far and they usually get confirmed within an hour or 2... I think the longest wait so far was 93minutes... but YMMV
Also, as mentioned, the fees are an inexact science and don't really guarantee any sort of confirmation time frame... just because you put a "priority" fee on a transaction, it doesn't obligate the miners to process your transaction in the next block... or ever.