I've tried to do 3 transactions in the last 24 hours, but they keep getting sent back. For the last attempt I chose the 2nd highest fee option that I could set, which clearly still wasn't enough. I'm assuming the highest fee option still won't be enough?
How can I adjust the maximum fee option? And what size fee would be enough for my transaction to get through?
First, make sure you are running the latest version 2.8.2 . This fixes some issues with fees in older versions.
You should be using the Dynamic Fees option which is very accurate at choosing fee sizes. Moving the slider more to the right will speed up confirms and it will estimate how many blocks it will take for a confirmation.