The highlighted "size" is what you need to multiply by the value recommended on http://bitcoinfees.21.co to get a rough idea of what you need to pay to get a quick confirmation. The fee shown on that dialog, is what Electrum plans to use.
This "size" is mostly a function of the number of "Inputs" required to add up to the amount you want to send+fee... each input will generally add around 148 bytes. (Note: each output will add around 34 and there is a fixed 10 bytes per transaction). In the example shown, 3 inputs (3 * 148) + 1 output (34) + 10 = 488 bytes.
So you can see, that if you had say 10 inputs, and 1 output... your transaction size is: (10 * 148) + 34 + 10 = 1524 bytes... given current recommended fees of 390 sats/byte, you get: 1524 * 390 = 594,360 satoshis or 0.00594360 btc...
If you have a lot of small inputs (ie. you have been collecting tiny amounts of faucets, clouding mining, captcha solving etc), you can quickly end up with inputs that are worth less than they cost to send!
As an example, the "fee" for each input you put into a transaction is essentially: 1 input @ 148 bytes * 390 sats/byte = 57,720 satoshis or 0.00057720 btc. So, at the moment, any input that is smaller than 0.00057720 would cost you more to send than it is worth!
Obviously, you can adjust the fees to suit (or do them manually and choose lower fees - NOT RECOMMENDED!!), but if you want your transactions to get confirmed quickly... you need to pay the fees required
