It doesn't show the exact size... but you can roughly calculate it using the standard formula:
the size of the tx will be: 180 bytes per input + 34 bytes per output + 10 bytes.
For determining how many inputs you are going to have... From what I have seen, it would appear that Mycelium tries to be smart and optimises the transaction size by using the most appropriately sized, oldest, unspent input(s)... if that is what you are asking?
For example... if your current wallet inputs are (in order from oldest to newest):
1. 0.01
2. 0.0043
3. 0.02
4. 0.1
5. 0.05
and you wanted to send 0.04BTC... it would use 5... which would *should* theoretically keep the transaction size (and fees required) as small as possible...
NOTE: I am not 100% sure that this is how it works... as I haven't checked the code either... but a quick look at my sends, seems to indicate that it is picking up the "best fit" unspent inputs to cover the outgoing amount.