The fees are lower during weekends, so you may try send all your coins to yourself in saturday or sunday in order to merge them and get lower cost on the following transactions.
To calculate the transaction size in bytes you can use the following formula:
inputs plus 180 + outpouts plus 34 + 10 plus or minus inputs for standard public keys or inputs plus 148 + outpouts plus 34 + 10 plus or minus input for compressed publick keys.
And then use some webpage that shows transactions fees per byte like http://bitcoinfees.21.co/ to calculate the proper fee.